摘要:

Carbon Monoxide Flame Bands BY R. N. DIXON Dept. of Chemistry The University Sheffield 10 Received 1st January 1963 The carbon monoxide flame bands have been photographed at high dispersion. The bands in the wavelength region 3100-3900 8 show a regular pattern. This is interpreted in terms of emission from a BZ level of bent C02 to high vibrational levels of the linear ground state the vibrational pattern being complicated by the effects of Fenni resonance. The upper state has an energy of about 46,700 cm-1 above the lowest level of the ground state and an equilibrium angle of 1 2 3 f 3 O . The rotational structure breaks off from which it is deduced that the upper state is probably 1B2. The light emission from the combustion of carbon monoxide has been the subject of numerous investigations.The spectrum consists of a complex series of narrow bands overlaid by a characterless emission that appears continuous at moderate resolution. The relative intensities of the two spectra vary with experi-mental conditions and the bands are favoured by low temperature and pressure. The complexity of the banded spectrum rules out a diatomic molecule as the emitter. Gaydon 1 has reviewed the subject and it is now generally accepted that the bands arise from the chemiluminescent reaction of carbon monoxide and oxygen atoms. However there has been no direct proof of this suggestion neither is there agreement over the nature of the electronic states of CO2 involved in the transition. This paper gives an interpretation of a high-resolution spectrum of the bands photographed using an afterglow source.EXPERIMENTAL Commercial carbon dioxide was partially dissociated in a Wood’s tube drawing an 8.c. current of 1.2 A at 2.5 kV. The electrodes were water-cooled aluminium alloy. The effluent from the discharge was pumped by a 150 l./min rotary pump through a light trap and down an afterglow tube 3 m long and 2 cm diam. A pale blue-grey glow filled the body of the tube and at the highest powers available a thin straw-coloured glow was seen on the wall of the tube. When viewed through a direct-vision spectroscope light could be seen throughout the whole of the visible range of the spectrum. The afterglow stretched a considerable distance down the tube and at a pressure of 1 mmHg the gas was still glowing as it entered the pump.The tube acted as a light pipe and the emission through the quartz end window was sufficiently intense to give a good photograph of the carbon monoxide flame bands with 5 min exposure on Ilford Zenith Astronomical plates using a Hilger small quartz spectrograph with a slit-width of 25p. The appearance of the spec-trum was very similar to that described by Hornbeck and Herman,2 who used a cool flame, and showed a system of partially resolved bands extending from the 3064A OH band to the long wavelength limit of the plates at about 48OOA. A plane mirror mounted inside the pump-end of the tube perpendicular to the tube length was found to increase the light output by about one-half. The maximum light output with the power available was obtained with a pressure of 5 mm Hg.The spectrum was then photographed in the second order of a 15,000 line/inch concave grating spectrograph. Exposure times of several days were required. Since the material sputtered from the electrodes slowly poisoned the glassware the discharge tube was cleaned with 4 % HF solution every 12 h and the afterglow tube every 2 days. The best plates 10 106 CARBON MONOXIDE FLAME BANDS were taken with an exposure time of 5 days using a slit-width of 35p and pre-exposed Kodak OaO plates. Instability in the temperature and pressure in the spectrograph over such Iong periods resulted in a slight blurring of the focus and the best resolution obtained was about 0-5 cm-1. Wavelengths were measured on a photoelectric comparator with reference to an iron hollow cathode lamp using the M.I.T.Wavelength tables 3 and the vacuum corrections of Edlen.4 RESULTS The high-resolution photographs of the carbon monoxide flame bands show a large number of narrow bands and close groups of lines every few A apart. The intensity distribution in the 3064A OH band corresponds to a low rotational tem-perature. The most striking feature of the spectrum is the occurrence of two types of pairs of strong violet degraded bands in the wavelength range 3100-3900 A. One such type has a spacing between heads of 30-40cm-1 and the shorter wavelength head is the stronger in each pair; the other type has a spacing between heads of 60-80 cm-1 and the longer wavelength head is the stronger in each pair and shows a splitting of 2-3 cm-1 (see plate 1).These features may be arranged in series of up to 5 members of similar type with a spacing of -300 cm-1 and the complete spectrum shows an alternation of series of narrow pairs and series of wide pairs. At longer wavelengths than 3900 A this regularity is no longer obvious. DISCUSSION Recent measurements 59 6 on the kinetics of the reaction between carbon monoxide and oxygen atoms suggest a mechanism : CO('C*)+ 0(3P)+ M-*CO,*+M (1) with an activation energy of 4-10 kcal/mole. The dissociation energy Dg of the ground state of C02 to ground state CO and 0 is 5.435 eV.7 Since the shortest wavelength bands lie at about 3100A (4 eV) this mechanism requires that the lower state of the emission lies no higher than 2 eV above the ground state. Mole-cular orbital considerations predict 8 * 9 that the lowest excited states of C02 should be 1B2 and 1Az states at about 5 eV and 3B2 and 3A2 states at about 4 eV in all of which the molecule will be bent.(Following Mulliken's recommendations 10 for the axes of C2v molecules a vector parallel to the 0-0 distance has species b2.) No experimental evidence has been found for any electronic absorption of C02 at longer wavelengths than 2500A (5 eV) even with absorption paths as great as 280 m atm.11 It is therefore concluded that the bands must arise from transitions from a bent upper state to high vibrational levels of the linear ground state the extensive wavelength range of the spectrum being the result of the change in geometry. A bent COz molecule with an angle greater than about 110" will be a slightly asymmetric prolate top with the top axis parallel to the 0-0 distance.The rota-tional levels will therefore be closely approximated by -F(J K ) = BJ(J+ 1) +(A-@K2; = 3(B+ C). (2) The asymmetric top splitting of the double degeneracy of levels with K>O will be greatest for K = 1. The zero nuclear spin of 160 atoms will result in the existence of only one of the levels for each J and K but this will alternate between the upper and lower levels of the K doublets for increasing J and constant K. The rotational levels of the linear ground state are given by F(J) = BJ(J+I). (3 PLATE 1.-The carbon monoxide flame bands. (a) and (b) the assignment of quantum numbers 2v1+u2 and n to series spectrum ; (c) a typical section of the spectrum at longer wavelengths R .N. DIXON 107 The bending vibration v2 is doubly degenerate and levels with v2>0 may possess angular momentum Zh12n about the molecular axis where I = 02 212-2 * 1 or 0. There is a strong Fermi resonance between v2 and the symmetric stretching vibration vl due to the near equality of v1 and 2v2 and this results in a separation of levels differing in I which are degenerate in a harmonic approximation. The close pairs of bands in the spectrum may thus be identified with sub-bands differing in K (K EZ). Since the spacing in the pairs is not constant and no constant combination differences can be found in the spectrum it is concluded that the transitions are of type a (parallel in the limiting symmetric top). The single-headed appearance of most of the bands is in keeping with the low relative intensity of the Q branch in parallel bands with low values of K.The pairs of bands are assigned as follows. (i) Narrow pairs spacing 30-40 cm-1. Sub-bands with K = 0 (longer wave-length) and K = 2 (shorter wavelength and stronger on account of the double de-generacy of levels with K>O). These will be referred to as Z and A bands, respectively. Sub-bands with K = 1 (longer wavelength, the splitting of -2 cm-1 being mainly asymmetric top splitting of the upper state rotational levels) and K = 3 (weaker shorter wavelength band). These will be referred to as II and @ bands respectively. Since the bands are degraded to the violet B'>B". For an angle of 120" this requires that the upper-state bond-length shall not be more than 0.15 A longer than that in the ground state.It is now necessary to enquire into the nature of the spacing of -300 cm-1 between members of the series of similar bands. The vibrational levels of C02 have been extensively studied by Courtoy.12 For each " polyad " of levels that are nearly degenerate in the harmonic approximation and have constant values of I u3 and (2111 + 02) the splitting between levels increases almost linearly with (201 + 02). Extrapolation of this behaviour suggested that a splitting of -300 cm-1 would be expected for levels with (2ul+u2) = 20. Courtoy has shown that the levels with ( 2 ~ + 212) up to 8 are represented within the experimental error by using a diagonal energy level expression : (ii) Wide pairs spacing 60-80 cm-1.G,(u1 212 213 1) = 1345.0401+ 667.25212 +2361.7103 - 3.63~ +3*44tl,tl2 - 19.280103 -0.63521; - 12.510~0 - 12.5603 +0*130; -0.080f0 +O-O211~11~21~ +O*Olt.$ - 0 * 0 7 ~ ~ +0*015~ +0*07O1V$ + 0 * 0 1 0 2 ~ ~ +0*775Z2. (4) The Fermi resonance is then evaluated using off-diagonal matrix elements of the anharmonicity : W(V~ 212 03 2; 01 -1 02 +2 03 I ) = (51*31-0*1501 - 0 4 1 ~ 2 - 0*7803)$[(0 + 2) 2- Z']'U~. ( 5) The calculation of the energy levels is carried out by diagonalizing a square matrix of order [ul+ 1 +$(vz- Z)] for each polyad. A programme was therefore prepared to evaluate the energies of levels with high values of (2211 +uz) using the Manchester University Mercury Computer. A trial run for (201 + 212) = 20 gave a maximum separation of E:g" levels of 296 cm-1.The calculations were then extended to give energies of all the levels with 20<(2ul+v2)<30 03 = 0 and 0<1<4. A portion of these calculations is illustrated in fig. 1 108 CARBON MONOXIDE FLAME BANDS This method of calculation is subject to two sources of error. (i) The extrapolation of a series of vibrational levels of low quantum number to give levels of high quantum number is an unreliable procedure in any molecule. In this case it may be justified by the small values of the cubic coefficients in eqn. (4), and the fact that the highest calculated energy levels are at about 45 % of the energy of the first (spin-forbidden) dissociation process and have bending vibrational amplitudes of about 60”. 1 I - I a3 22 24 2e (2Vl +v2) FIG.1 .-A portion of the calculated array of vibrational levels for the ground state of C02 v3 = 0 2 ~ 1 + ~ 2 = 20. . . 26 Z = 0 2 4. z=o 1 = 2 - 1 = 4 (ii) The theoretical expressions on which eqn. (4) and ( 5 ) are based are derived using second-order perturbation theory to evaluate the anharmonic mixing of levels which differ in the value of (2ul+212).13 The values of the appropriate matrix elements have been estimated by substituting Courtoy’s data 12 for the vibration-rotation interaction constants in Dennison’s equations,l3 and it was found that this approximation is no longer strictly valid. In addition it is seen from fig. 1 that the resonance leads to an overlapping of polyads which differ in the value of (201 +u$. However preliminary estimates of the accuracy of the calculations indicate that the calculated energy level scheme is not seriously in error.In particular the separation of neighbouring levels which differ only in 2 which is as small as 10 cm-1 in some cases is thought to be quite accurate R. N. D ~ x O N 109 All the strong bands between 3100 and 3900A and most of the weak bands, may be explained with the aid of these calculated lower state vibrational levels if it is assumed that all the transitions arise from the various rotational levels of one upper state vibrational level of species B2. In particular the following details of the pattern of bands is reproduced. (a) The separation of the levels in a polyad is greatest between the two highest levels and each interval is 25-30 em-1 less than the next highest interval for levels in the upper half of the polyad.(b) The maximum separation in each polyad increases by 15-20 cm-1 with an increase of 2 in (2vl+ 4. (c) The alternation of series of narrow and wide pairs of bands is associated with even and odd values of (2211 + 24. The nature of this patttern of levels makes possible the assignment of vibrational quantum numbers to the bands. This has been done by fitting the calculations to the experimental pattern for the shortest wavelength bands which involve the lowest value of (2ul+ 02). The comparison is given in tables 1 and 2. Since the harmonic quantum numbers 01 and 112 are meaningless for such strong Fermi resonance the levels are labelled by the value of (2vl+ 112) and a running number n that has the 201 +v2 22 22 22 24 24 24 24 26 26 26 26 26 28 28 28 28 2 V l + v2 23 23 23 25 25 25 25 25 27 27 27 27 27 29 2 3 4 1 2 3 4 1 2 3 4 5 2 3 4 5 3 1,011 -6 3 1,300-6 31,558-4 29,211.7 29,533-5 29,833-7 30,108.0 27,716.5 28,051-7 28 3 65.9 28,6584 28,927-9 26,567-5 26,8951 27,200.8 27,48 5.2 15,674.8 15,388-1 15,122.8 17,4703 17,155.4 16,858-3 16,580.7 18,961.5 18,638-6 18,332.4 18,044.3 17,776.3 20,123.5 19,809-6 19,s 12-5 19,233-8 TABLE THE ASSIGNMENT OF rI BAND-HEADS n Yobs.(cm-9 G&* (cm-1) 2 3 4 1 2 3 4 5 1 2 3 4 5 5 30,279-5 30,572-9 30,844-3 28,4703 28,799-2 29,107.0 29,393-2 29,656-1 26,973.8 27,317.4 27,637.0 27,937.1 28,2159 26,770.0 16,411.5 16,118.8 15,846-0 18,213.3 17,893.7 17,591.3 17,307.5 17,043.9 19,704.8 19,378.1 19,067.4 18,773-9 18,499.1 19,959.9 Y + G" (cm-1) 46,686.4 46,688.7 46,68 1 a 2 46,682.2 46,68869 46.692.0 46,688-7 46,678-0 46,690.3 46,698.3 46,702.7 46,704-2 46,69 1-0 46,704-7 46,713-3 46,7 19.0 Y + G" (cm-1) 46,691-0 46,691.7 46,690.3 46,68 3 * 8 46,692.9 46,698-3 46,700-7 46,700.0 46,678-6 46,695-5 46,704.4 46,711.0 46,715.0 46,729-110 CARBON MONOXIDE FLAME BANDS value 1 for the upper level in each polyad.This vibrational assignment is not unique. The values of (2ul+v2) cannot be reduced unless the energy level cal-culations are considerably in error since the chosen numbering assigns some bands to the uppermost levels of polyads.However all the bands could have (2ul+272) increased by 4 with a corresponding increase of 1 in n. The chosen numbering gives vo = 46,700k 20 cm-1. It is difficult to estimate the probable error in the calculations, but this might be several hundred cm-1. The lack of exact agreement between cal-culation and experiment indicates that the calculations are not completely reliable. The upper level therefore lies 8 & 1 kcal/mole higher than the energy of CO(lC+) + Further justification for the above model may be found in the intensity distribu-tion. The Frank-Condon principle predicts a long progression in the bending vibration for a transition from a bent upper state to a linear lower state.Fermi resonance will complicate this distribution. The wave-functions of the polyad of X levels with (2vl+272) = 8 have been calculated with the approximation that all anharmonic effects have been neglected except the Fermi resonance and with 01 = 2w2. From the form of these functions it is possible to generalize the highest level of each polyad corresponds to a classical motion with a turning point at which the molecule is bent with elongated bonds; the lowest level has a turning point at which the molecule is bent with contracted bonds; and the intermediate levels are intermediate in character. The observation that the bands involve only the upper levels in each polyad indicates that the excited state equilibrium bond length is greater than that in the ground state.The frequency difference between adjacent-X and A bands is the sum of the upper state rotational energy difference 4(A’-B‘) and the separation of the lower levels. The calculations show that this lower state separation is -10 cm-1 and the upper-state difference is therefore the greater part of the observed spacing. Thus we can use the expression : to give a good estimate of (A’ -3’). A similar expression will give 8(A’ - B’) frem the observed spacing of Il and bands. The values of 4(A’-B’) and 8(A’-B’) calculated in this way are given in tables 3 and 4. The consistency of the various o(3~). 4(A’ - B’) = AvobS(A - C) - AG&,,(C -A) ( 6 ) I TABLE THE DETERMINATION OF (A’-) FROM THE SPACING OF PAIRS OF AND A BAND-HEADS 2 U l + 02 n Avobs.(cm-l) AG”c,lc.(m-9 4 ( ~ - B? (cm-1) 22 22 24 24 24 24 26 26 26 26 26 28 28 2s 28 2 4 1 2 3 4 1 2 3 4 5 2 3 4 5 33.3 41.7 30.5 31.8 34.6 41-0 294 31.1 32.6 35.6 40.5 30.2 31.8 33-4 37.3 13-3 21-2 10-6 12.3 14.8 18-7 9-9 11.5 13.6 16-7 21-9 10.8 12-6 15.2 19.3 20.0 20.5 19.9 19.5 19.8 22.3 19-9 19.6 19.0 18.9 18-6 19.4 19.2 18.2 18. R. N. DIXON 111 TABLE 4.THE DETERMINATION OF (A'-') FROM THE SPACING OF PAIRS OF n AND @ BAND-HEADS * 2Ul +v2 23 23 23 23 25 25 25 25 25 27 27 27 27 27 29 n 1 2 3 4 1 2 3 4 5 1 2 3 4 5 5 (Cm-') 62.7 66.3 71-8 76.0 61.2 64.1 68.2 74.1 83.3 60.2 62-6 65.5 70.2 77.4 72.4 AG"dc (cm-1) 22.7 25.4 30.7 38-7 20.4 23-6 28-1 34.7 45.2 19.3 22- 1 25.9 31-5 39.9 35.8 S(X- B')(cm-1) 40.0 40.9 41-1 37.3 40-8 40.5 40.1 39.4 38-1 40.9 40.5 39.6 38.7 37.5 36.6 * The mean frequency of the double 17 band-heads are used in this calculation.values is extremely good. Since the separation in energy of lower state levels that differ in I is mainly due to the same anharmonic terms that cause the splittings within the polyads and the vibrational structure has been cksely fitted the value of A' should be accurate to within 10 %. Hence (A'-B') = 4-9kO-5 cm-1. If we assume that the upper-state bond length is 0.1 A longer than that in the ground state (the bands are violet degraded hence the increase cannot be more than 0.15 A), corresponding to B' = 0.412 cm-1 then the angle OCO = 123+3".The value of A' indicates that bands should be observable for K up to about 9 if the rotational temperature is close to room temperature. Bands with K = 4 are found at the expected frequencies close to each strong X A pair of bands but no K = 6 bands are found. A narrow group of lines is found at the expected frequencies of K = 5 bands and no K = 7 bands are found. The rotational structure of the upper state must therefore break off at about K = 5. The possible electronic states arising from the combination of ground state CO and 0 into a CO2 molecule of point group C2v are : On simple valence grounds none of these states would be either strongly bonding or strongly antibonding.Hence the stability of the lowest 3A2 and 3B2 states predicted by molecular orbital theory must arise from strong mixing with bound states from the products CO(3II)+O(3P). It would therefore be surprising if the lowest 3B2 state dissociates into ground state products via a potential maximum of 8 kcal/mole. Thus the upper state of the bands is probably 1B2 rather than 3B2, and the breaking-off is due to predissociation by a repulsive triplet state made possible by spin-orbit interaction. Since the 1B2 state arises from the removal of the degeneracy of the TC&~ lAu state of linear C02 on bending the flame bands are thus associated with the absorption spectrum of C02 at about 1500A.9 The straw-coloured glow on the surface of the glass indicates that some excited molecules are formed in a wall reaction. This glow is about 1 mm thick at a pressure of 1 mm Hg and the application of random-walk equations indicates a mean life-time of 3 x 10-4 sec or 4000 collisions. Since this will be a lower limit to the radiative lifetime the straw-coloured glow probably involves an excited triplet state 112 CARBON MONOXIDE FLAME BANDS Thus the emission from the carbon monoxide + oxygen atom reaction would appear to involve both singlet and triplet states. I am indebted to Mr. L. Faine for assistance with the construction of the grating spectrograph and to Mr P. Blundell and Mrs. A. Fairburn for assistance with the calculations. 1 Gaydon The Spectroscopy of Flames (Chapman and Hall London 1953 chap. 6. 2 Hornbeck and Herman Nat. Bur. Stand. circ. 1954,523,9. 3 Harrison M.I.T. Wavelength Tables (Why New York 1939). 4Edlen J. Opt. SOC. Amer. 1953 43 339. 5 Clyne and Thrush Proc. Roy. SOC. A 1962,269,404. 6 Mahan private communication. 7 Rossini Wagman Evans Levine and Jaffe Nat. Bur. Stand. circ. 1952 500. 8 Walsh J. Chem. SOC. 1953,2260. 9 Mulliken Can. J. Chem. 1958 35 10. 10 Mulliken J. Chem. Physics 1955 23 1997. 11 Callomon private communication. 12 Courtoy Can. J. Physics 1957 35 608. 13 Dennjson Rev. Mud. Physics 1940,12 175

ISSN:0366-9033    

DOI:10.1039/DF9633500105

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Spectrum of the C3 Molecule BY L. GAUSSET,* G. HERZBERG, A. LAGERQV~ST AND B. ROSEN Institut d’ Astrophysique, UniversitC de Likge National Research Council, Ottawa, Canada Institute of Physics, University of Stockholm Received I st February, 1963 High resolution spectra of the 4050 group of the C3 radical have been obtained in absorption in the flash photolysis of diazomethane. A reinvestigation of the 0-0 band at 4050A shows the electronic transition to be of the type lTI[,--lzt (not lCi-lilg). From the analysis of several additional bands, especially 000-020 and 010--010, it follows that the bending frequency v2 of the ground state is very low (about 70 cm-1). Because of this low value of v;, a very large /-type doubling in the 010 level arises. For the same reason the rotational constant a; is very large.The low value of vg combined with large vibronic (Renner-Teller) splittings of the levels involving v i in the upper state (vh>v;) is largely responsible for the complexity of the vibrational structure of the 4050 group: since even at room temperature v; is excited by several quanta, both progressions and sequences in v2 arise which, on account of the vibronic splittings, do not show the usual simple regularities. The spectrum of the C3 molecule (the so-called 4050 group) was first correctly identified by Douglas.1 He analyzed the main band at 4050A and showed that the C3 molecule is linear and symmetrical in both upper and lower state and that the C-C distance in the lower state is 1.28 A. He was, however, unable to decide whether the electronic transition is 1II-lZ or IZ-lIn, but gave the latter alter- native a slight preference.The reason for the difficulty to decide unambiguously between the two alternatives was connected with the fact that in emission the rotational temperature is fairly high and the lines near the band origin are strongly overlapped by the higher lines of the R branch; in addition, a slight perturbation occurs which makes it difficult to reach a definite conclusion. The other bands of the 4050 group have thus far not been definitely analyzed, mainly because of the overlapping of a large number of bands (compare the attempts by Kiess and Bass 2 and Kiess and Broida 3). When, therefore, in the study of the flash photolysis of diazomethane the C3 spectrum was observed in absorption under low temperature conditions, it appeared of interest to try to produce as complete a spectrum as possible, hoping that it would lead to a better understanding of its structure.While this aim has not yet been reached, a sufficient number of new results has been obtained to warrant a preliminary report. EXPERIMENTAL The apparatus used was entirely similar to that described by one of us 4 for the study of CH2 and CH3. By the use of multiple traversals, an absorbing path of up to 64m was attained at a partial pressure of the CHzNz of 0.04 mm. In most experiments the CH2N2 was mixed with a great excess of N2 in the ratio 103/1. The presence of the inert gas ensured a low temperature rotational distribution and, it was hoped, would also eliminate all or most vibrational transitions with 0:’ f 0.The first spectra were taken in the second order of the N.R.C. 21 ft. grating. Except for the expected reduction of the rotational *Aspirant8 of the BeIgian National Foundation of Scientific Research. 113I14 SPECTRUM OF THE c3 MOLECULE temperature, no significant simplification of the absorption spectrum compared to the emission spectrum was obtained. In order to make further progress, therefore, new spectra were taken first with the N.R.C. 35 ft. spectrograph, and finally with the new 9-6 m Ebert spectrograph in the 14th order of a 7500 lines/in. Harrison grating with a reciprocal dispersion of 0.1 A/mm and an attained resolving power of 300,000. These spectra have made it possbile to analyze unambiguously a number of bands in addition to the main band.While a complete understanding of the vibrational structure has not yet been reached, a number of points concerning this structure have been established. RESULTS AND DISCUSSION BANDS WITH THE (OoO) LEVEL OF lx: AS LOWER STATE The re-investigation of the main band at 4050 has essentially confirmed Douglas's original analysis but has made it quite definite that the transition is lrIa-lZ$ and not 1C - Ing. On the basis of molecular orbital theory, a 1x2 is indeed expected as the ground state, and a Ina is easily accounted for as a low excited state (see Mulliken 5). In table 1 the B values are given for the main band as well as for several other bands. There is an appreciable A type doubling in the 'nu state which increases fairly regularly with J(J+l).The value of the splitting constant q is also included in table 1 . TABLE ROTATIONAL CONSTANTS OF BANDS OF THE 4050 GROUP 2 (A) 4102 4073 4073 405 1 4039 4019 4005 3993 3984 3972 3950 3950 3936 3936 3929 3916 YO (Cm-1) I* 24389 1 24543 6 24544 5 24676 10 24749 5 24872 4 24949 2 25044 6 25093 4 25168 4 25308 3 25310$ 3 25396 3 25398 3 25441 3 25529 3 vibronic transition t tme assignment (Ti, -4;) 001)-040 1II, -1z; 000-020 TI,, -a; 000-020 mu -1Z; Ooo-000 1z; -lnu 010-010 1z; -In, 010-010 1 q -mu 010-010 lAg -Inu 010-030 (1A -In) (030-030) 1X; -1nu (030-010) 040 B" qb - - 0.4557 - 04466 - 0.4298 - 0.4401 0.004, 0.4422 0.006 0.463 0-008 0-452 ? 0.4424 0.005 0.4420 0.005 0.456 - (0446) - 0.456 - 0.446 - 0.4297 - * estimated intensity.t assignments in parentheses are uncertain. $ only the Q branch has been found. The other bands which have been analyzed are either of the same type as the main band or of the types I I I - I A , IA-lII, 1X+-1IT, 1ZC--1II, as indicated in table 1. AlI have strong Q branches. As expected, since the main band is of the 1 type, no bands of the 11 type (E- E, II-II, . . .) have been observed. The bands of type II- A and A-II have six branches, but the doubling of the A states has not been resolved. The bands of types E+-n and 2-I3 can be distinguished by establishing whether odd or even lines are missing in the Q branches (and cor- respondingly even or odd lines in the P and R branches).L. GAUSSET, G. HERZBERG, A . LAGERQVIST AND B. ROSEN 115 Since the molecule is linear and symmetrical in both the upper and lower state, according to the Franck-Condon principle, only the totally symmetric vibration v1 can be excited strongly in the transition, i.e., Avl = 0, & 1, +2, .. . . For the other (non-totally symmetric) vibrations, transitions with Avz = 0, +2, +4, . . . and At73 = 0, +2, +4 are possible, but all transitions except those with Av2 = 0, At13 = 0 should be very weak. An exception would arise only if there were Fermi resonance between u1 and 2 ~ 2 , or if ui differed from v$’ by more than, say, a factor of 2, when transitions with At72 = +_2 could also be strong. If the spectrum of C3 followed the simplest scheme, we should observe in “ cold ” absorption a single progression in v { and nothing more.The fact that the actual absorption spectrum observed under low temperature conditions is not so simple can be accounted for either by the occurrence of strong “ hot ” bands or by the superposition of several electronic transitions. While a number of low-lying electronic states are predicted for C3, it appears wise first to attempt to explain the whole of the observed absorption bands by a single electronic transition on the basis of the first alternative (strong presence of hot bands). We shall disregard here the second alternative without in any way considering the absence of other electronic transitions as settled. One band has been found, at 3916 A, which has the same lower state as the main band and is of the same type. It is probable that this band is the second band of the expected progression in v i which would then have the frequency 853 cm-1.However, there are two other bands of similar appearance at 3907 and 3929 A whose analysis has not yet been accomplished.* If these two bands also have the same lower state, then it appears likely that the three upper vibronic II states correspond to Fermi resonance between v ; and 2vi (or possibly 4~5). Here it must be realized that in an electronic I3 state, 2v2 (and similarly 4v2) gives rise to two I3 states on account of vibronic (Renner-Teller) interaction. If this interpretation is correct, all one can say about v { is that it is in the neighbourhood of 853 cm-1. The A type doubling in the upper state of the 3916 band is much larger than in the upper state of the main band.BANDS WITH THE (010) LEVELOF 1z: AS LOWER STATE An important band for an understanding of the 4050 group has been the band at 4019 A. This band consists of six very clear branches : two P, two Q and two R branches. It must be interpreted to be vibronically of the 1A-1IT type. A large doubling in the lower (In) state is observed (q,-0.006). There can be little doubt that this band is one of the 010410 bands. In the upper state the 010 level has three component levels Z+, Z- and A. We believe that the 4019A band involves the A state of this group. The large doubling in the lower state must be 2-type doubling since there is no electronic angular momentum in the lower state. The magnitude of q in a linear molecule is approximately given by B~/co. Substituting the observed values of q and B, then ~ 0 z 4 0 cm-1, i.e., an extremely small bending frequency in the ground state.While such a low bending frequency may appear unreasonable, it is not so surprising since the rigidity of linear molecules like CO2 is mainly deter- mined by the four electrons in the ng orbital. NCO (and similarly B02) with one * Note added in proof-The bands at 3950, 3936 and 3929 8, have now been analyzed. They are all of l f i u - l z $ vibronic type. The first two seem to have the same lower state (020) as the band at 40738,. The upper state of the bands at 3950 and 39298, appear to be the same, while the 3936A band has the same upper state as the 39168, band previously discussed. In addition, a band at 4005A of 1Ei-*17u type has been analyzed. These results have been included in the revised table 1.116 SPECTRUM OF THE c3 MOLECULE less electron in this orbital has an appreciably smaller bending frequency than CO2, and C3 has no electrons in this orbital in the ground state.The low value for v> would account for the occurrence of many '' hot " bands, which complicate the spectrum, even at room temperature. Two further bands, at 3972 and 3984 A, and possibly a third at 4039 A, have the same lower lTIu state as the 4019 A band. This state is probably the 010 state. The upper states of these bands are lE;, 1E:Q" and ~ E F , respectively. It is probable that the two states 1X: and 1E; are the remaining two states belonging to the 010 group in the upper state, of which the upper lAg state of the 4019 band is the third component.This interpretation would, however, require a very large and unsym- metrical Renner-Teller splitting in the upper state. BANDS WITH THE (000) LEVEL OF AS UPPER STATE Another band of considerable importance for the understanding of the spectrum is at 4073 A. This band shows a much wider spacing than most of the other bands, indicating a much greater difference B'-B". Closer analysis shows that it consists of two sub-bands, each of which has the same upper state as the main band at 4050 A. This conclusion is supported by a very good agreement of the upper state com- bination differences. The two lower states appear to be 1E: states. They are separated by only about 1 cm-1 and are about 140 cm-1 above the lower state of the main band.The only low lying vibrational levels in the ground state that can combine with the 000 upper state are the levels 1Et and lAg of 020. The fact that 2v; in this way comes out to be only 133 cm-1 is in striking agreement with the conclusion from the interpretation of the 4019A band that v i is exception- ally small. A further confirmation is that B" for the 4019 A band is approximately half-way between B" for the main band 4050 A and B" for the 4073 A bands. The only question is why are there apparently two 1x4" states instead of one lEg and one lAg, i.e., why is half the lAg state missing. There are certainly a number of unidentified lines, but it has not been possible to analyze them. They may be the other half of the lAs state. A weak band at 4102A has an even greater difference B'-B" and is probably one of the bands 000440.Unfortunately, the P and R branches of this band are too weak for a reliable analysis, but the intensity dis- tribution and the spacings in the progression OOo--OOO, 000-420 and 000440 are reasonable and in favour of the suggested assignment. The two bands 4073 and 4102 are the only prominent bands on the long wavelength side of the main band, while all other bands are on the short wavelength side, as expected, if it is assumed that v; is much larger than v i , and that only ~2 is excited in the lower state. OTHER BANDS Finally, a band at 3993 A was analyzed and found to be a 1A-ll-I band. How- ever, its lower state is not identical with that of the 4019 A band. This band may be one of the 030-030 bands since 030 in the lower state is the next level above 010 that gives rise to a lITu vibronic state. However, the B" value does not agree well with this interpretation, and it is difficult to account for the fact that the 020-020 bands have not been found; but many features remain to be analyzed or are too complex to be analyzed.CONCLUSION In fig. 1 a provisional energy level diagram of the vibrational levels in the upper and lower electronic state of the 4050 group is presented. The exact positions ofL. GAUSSET, G. HERZBERG, A. LAGERQVIST AND B . ROSEN 117 the upper and lower states of the 010-010 and 030-030 groups relative to OOO--OOO have not been established. The diagram makes clear, however, some of the reasons for the complexity of the vibrational structure of the 4050 group. Because of the smallness of v;, several quanta of this vibration are thermally excited even at room temperature and give rise to both progressions and sequences in 212. Because of C m-' 2550C 2500C f' I0 oto 2 0 ---- -,ii- t n u l nu dr i,o,ol+ b,4,0] FIG. 1.4bserved energy levels of C3. This diagram is based on preliminary results. Table 1, as corrected in proof, contains several new results which the reader can easily incorporate in the diagram (see also the note added in proof). the Renner-Teller splittings in the upper state, the sequences do not follow the usual simple regularity. While much remains to be done in order to elucidate all of the complexity of the spectrum, we believe that some progress has been made toward this aim. We are greatly indebted to Mr. J. Shoosmith for his effective help in taking the spectrograms on which this work is based and to Dr. A. E. Douglas for critical comments. 1 Douglas, Astrophys. J., 1951, 114, 466. Clusius and Douglas, Can. J. Physics, 1954, 32, 319. 2 Kiess and Bass, J. Chem. Physics, 1954, 22, 569. 3 Kiess and Broida, Can. J. Physics, 1956, 34, 1471. 4 Herzberg, Proc. Roy. SOC. A, 1961, 262, 291. 5 Mulliken, Can. J. Chem., 1958, 36, 10.

ISSN:0366-9033    

DOI:10.1039/DF9633500113

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Evidence for a Double-Minimum Potential in an Excited State of C102 BY J. €3. COON, F. A. CESANI AND C. M. LOYD Physics Dept., Texas A. and M. University College Station, Texas, U.S.A. Received 21st January, 1963 Evidence is presented which suggests the existence of a double-minimum potential in the anti- symmetrical vibrational co-ordinate of an excited electronic state of C102. A previously reported vibrational analysis of the 36008, absorption system of C102 vapour proposes the excited state assignments 2 v j = (1+-Of) = 1559 cm-* and 4~5-2~5 = (2+- 1 +) = 16405 cm-1. In the present work, a two-parameter doubleminimum potential function is adjusted to fit these levels of the QS mode. The resulting function has a barrier of 2520 cm-1, and at each potential minimum one bond is 0.065 A shorter and one bond is 0.065 8, longer than the average bond of 1.620 A.The iso- tope shift calculated for the interval (1+-0+) agrees with the shift observed; however, the shift calculated for (2'-1+) does not. A Franck-Condon calculation based on the vibrational wave functions of the double-minimum potential yields intensity ratios in approximate agreement with the experimental ratios. The isotope shifts and intensity ratios calculated assuming a harmonic potential in the Qj mode do not agree with experiment. Recently 1 an intense progression of vibronic bands in the 3600 A absorption system of ClOz has been given the assignment (ui, 0,2)+(0,0,0), and a less intense progression has been given the assignment (pi, 0,4)+(0,0,0). These progressions are labelled (c) and (d) respectively.The observed intensities of these progressions relative to the (a) progression (ui, O,O)c(O, 0,O) is much greater than is expected on the basis of a harmonic potential in the Q; co-ordinate, but a double-minimum potential explains the observed intensities qualitatively.1 Furthermore, the isotope shift of 2vi as obtained from progressions (c) and (d) is about 25 % less than that expected for a harmonic potential. The purpose of this paper is to explain quan- titatively the anomalous intensity and isotope shift in terms of a double-minimum potential function in the Q; co-ordinate. Mulliken 2~ and Walsh 26 have discussed the probability that not only C102 but some excited states of SO2 and NO2 differ slightly from C2v symmetry.Mulliken suggests that the 2bl molecular orbital is responsible for these slightly unsymmetrical states. 1 . THE POTENTIAL FUNCTION3 It is assumed that the potential function in the Q; co-ordinate has the form V(Q) = ?Q2 + A exp (- a2Q2) (1.1) 1 where Q is a co-ordinate satisfying the condition 27'= Q2. The minima of this three-parameter potential function are located at & Qm given by A parameter p is introduced by writingJ . B . COON, F. A. CESANI AND C. M. LOYD 119 A frequency vo is defined by L = (2ncvo)2, and a dimensionless parameter B is intro- duced such that the barrier height is Bhcvo. From eqn. (l.l), (1.2) and (1.3), (eP-p- 1) ep B ~ c v ~ = V(0)- V(Q,) = A (1 -4) The barrier height in cm-1 is and the minima are located at b =Bvo, Qm where We may replace the parameters 2, A and a2 by the parameters VO, B and p.In the present application p is set equal to 1.5. For this value of p the minima are para- bolic. The energy levels and wave functions of the potential function (1.1) have been determined 3 for 60 different barrier heights ranging from B = 0.0 to B = 6.0 in intervals of 0.1. Machine calculations based on the secular equation of the linear variation method lead to wave functions of the form 2 3 where the 4 k are harmonic oscillator wave functions corresponding to the frequency YO. Numerical values of the dimensionless energy levels G(O+)/vo, G(O-)/vo, . . ., G(4+)/vo, G(4-)/Vo are given for each value of the dimensionless barrier height B. Here G is the energy above the potential minima in cm-1.The coefficients ak are tabulated against B for each of ten levels Of, . . ., 4-. This requires ten tables of coefficients. The energy level tables are available 3 but the tables of ak are too extensive to publish. 2. BARRIER HEIGHT A N D BOND LENGTHS The data used in this section are taken from Coon and Ortiz,l 2 4 = G(1')-G(O+) = 1559 cm-l, 4vi-2~; = G(2+)-G(1+) = 1640.5 cm-l. Examination of the table giving the levels G/VO for a barrier B described in 6 1 reveals that only for B = 2-050 does the ratio agree with the experimental value, 1640.511559. This establishes the value of B. The value of vo is determined from the identity The result is vo = 1229 cm-1. value of vo yields the levels Multiplication of tabulated values of G/VO by this G(O+) = 945 cm-', G(1') = 2504 cm-', G(2') = 4143 cm-l, G(O-) = 1014 cm-', G(l-) = 3034 cm-', G(23 = 5128 cm-l.(2.4) Accordingly the (O+-0-) separation is 69 cm-1. It also follows that the barrier height is (2.5) b = Bv, = 2520 crn-l.120 DOUBLE-MINIMUM POTENTIAL I N cloz In order to calculate the configuration of the molecule at the potential minima, consider fig. 1. Let ro be the bond length at the symmetrical configuration and let r be the change of bond length corresponding to an antisymmetrical displacement Q. For small displacements the kinetic- e n e r i T 2mM ' = 2m sin2 O+M' For the excited state 26 is 107" 24'.4 Using 2T = Q: = p& M is given by 2T = p32, where (2.6) @2 and setting Q = em, (2.7) FIG. 1. Since p = 1-5, eqn. (1.6) gives (2; = 0.1414 x 10-40 g cm2.Consequently by eqn. (2.7), r, = 0.0651 A. Since the average bond length 4 for the excited state is 1.620 A, the bond lengths at the potential minima are The displacement of the potential minima from the symmetrical configuration is considered to be within the range of small displacements. bond lengths = 1.620 A & 0-065 A. (2.8) 3. THE ISOTOPE SHIFT The values of B and v g determined in Q 2 along with the assumed value of p are sufficient to determine a specific double-minimum potential function having the form of eqn. (1.1). The isotope shift calculated on the basis of this potential function and that calculated on the basis of a harmonic potential function may be compared with the isotope shift spectroscopically observed. Table 1 gives the isotope shift ACT = CT (C13502) -a(C13702) observed for the three band progressions, a(u) ( U ' , , O , O)+(O, o,o>, 40) <u;,o, 2)-+(0,0, 01, (3.1) 42.4 <u;,o, 4)+, 0,O).TABLE 1 . 4 % ~ ~ EXTINCTION COEFFICIENT E IN (l./mole cm) AND THE ISOTOPE SHIFT ACJ = g(C135O2) -~~(c1370~) FOR THREE BAND PROGRESSIONS IN THE SPECTRUM OF CHLORINE DIOXIDE 0: 2 3 4 5 6 7 8 9 (a) ucm-1 2242 5-5 231 19.2 23806.9 24488.2 25 1 64.3 25835.7 26502-4 27164-1 Aa cm-1 8 Aa cm-1 E Aa cm-1 e 89 23960.6 20.9 25571.5 32.0 139 233 24636.7 24.8 180 26232.2 35.9 168 384 25307.6 31.6 295 26888.8 40-4 147 24.1 491 25973.8 35.4 409 27540-7 44-9 106 29.6 638 26635.5 41.2 495 28187.9 50.1 37 34.5 900 27292.0 45.9 470 41-0 1076 44-9 1108J . B. COON, F. A. CESANI AND C. M. LOYD 121 These isotope shifts are obtained by averaging the shift measured by Urey and Johnston 5 with that measured by Ku.6 For each progression Ao may be plotted against v; and a best straight line may be drawn through the points.The interval between the straight lines for progressions a(v) and c(u) gives an approximate value for the isotope shift of 2v;, (experimental) A(2v;) = 11.6 cm? (3 -2) (2vJ21(2v3 5 = pi/P* (3.3) To calculate the isotope shift for a harmonic potential we may use The subscript i refers to isotopic quantities. The reduced mass p is given by eqn. (2.6) and /ii contains the mass of C137 instead of CW. Substituting 2v; = 1559 cm-1 into eqn. (3.3) leads to (harmonic) A(2v;) = 15.8 cm-', (3.4) which deviates considerably from the observed value. To calculate the isotope shift for the double-minimum potential it is noted that the barrier height BVO and the position of the potential minima +rm do not change with isotopic substitution.Consequently eqn. (1.6) and (2.7) yield Using these relations the parameters Bg and vb for the isotopic molecule may be determined and hence the energy levels may be obtained from the table of (Glvo). It follows that [G(l+)- G(O+)]i = 1547.6 cm-1 which compared to 1559 cm-1 corresponds to (3.6) in agreement with the observed isotope shift. However, the success of the double- minimum potential function in explaining the isotopic shift of the second observed interval is less marked : (PilP)+ = volvao = &/B. (3.5) (double min.)A(2v3) = 11.4 cm-', (expt.)A(4v; - 2 4 ) = 10.4 cm- ' (double &.)A[G(2+)- G(l')] = 26.0 cm-'.(3.7) 4. INTENSITY CALCULATION Another method of checking the validity of the double-minimum potential function of 92 is to test the ability of the wave functions of this potential to produce observed intensity ratios. Let &/a( 1 +, 0) designate the peak extinction coefficient divided by the frequency for a band of the progression c(u), and let e/a(O+, 0) designate the same for the corresponding band of progression a(v). According to the Franck- Condon principle, &/O(l+, 0) R(l+, 0) &/O(O+ -[-I y 0) - R(O+ 0) ' where R(u, 0) = Y'(v)'€"'(0)dQ3. (4.2) s In this overlap integral, Qj = Q;. The average value of the intensity ratio as determined from the data of table 1 is122 DOUBLE-MINIMUM POTENTIAL I N cloz The data for bands v; = 3,4,5 and 6 are averaged.This experimental intensity ratio is to be compared with the value calculated from eqn. (4.1). For the parameter B = 2.050 the tables described in 6 1 give the coefficients a k of the wave functions, ”’(1’) = 0.706 4; - 0.408 4; - 0.570 4; + . . . , Y’(0’) = 0-623 4h-tO.744 4;+0.241 4:+ . . . . (4.4) The 4; are harmonic oscillator wave functions corresponding to the frequency given by the double-minimum parameter vo = 1229 cm-1. The ground-state vibrational wave function is Y ( 0 ) = &, (4.5) where 4; is the harmonic oscillator wave function for 0’’ = 0 corresponding to the frequency v’j = 11 10.5 cm-1.1 The ratio R(1+, O)/R(O+, 0) is easily evaluated by use of the formulae, where r(v, 0) = J4&YQso [R(1+, O)/R(O+, 0)12 = 1.13, [R(2+, O)/R(O+, O)-y = 0.22, The result is (4.7) (4.8) which is in fair agreement with eqn.(4.3). A similar calculation yields which compares in order of magnitude to the intensity ratio of progression d(v) to progression a(v). These calculated intensities are a great improvement over those based on a harmonic potential in the Qj mode. Using vj = 1559/2 and v’j = 11 10.5 cm-1 the harmonic potential leads to 0.015 and 040036 in the place of values given in eqn. (4.7) and (4.8) respectively. DISCUSSION The double-minimum potential function specified by ( p = 1.5, B = 2-050, and YO = 1229 cm-I), having a barrier of 2520 cm-1, explains the two observed vibra- tional intervals 1559 cm-1 = If-Of and 1640-5 cm-1 = 2f- 1’. However, this potential explains the small isotope shift of only the first interval. Calculations not reported above show that a potential function with a barrier of about 3500 cm-1 is able to explain the small isotope shift observed for both intervals. For such a high barrier the second vibrational interval is reduced to 75 % of the value observed. The fact that both of the intervals and the isotope shifts of both intervals can not be explained by a single double-minimum potential function constitutes a serious difficulty.J . B. COON, F. A. CESANI AND C. M. LOYD 123 This work was supported by the United States Air Force under Contract No. AF49(638)-593 monitored by the AF Office of Scientific Research of the Office of Aerospace Research. 1 Coon and Ortiz, J. Mol. Spectr., 1957, 1, 81. ZaMulliken, Can. J. Chem., 1958, 36, 10. Ritchie, Walsh and Warsop, Spectroscopy (ed. Wells), (Pergamon Press, Oxford, 1962). 3 Coon, Naugle, Henderson and McKenzie, Report Air Force OSR (Contract 4 Coon, DeWames and Loyd, J. Mol. Spectr., 1962, 8, 285. 5 Urey and Johnston, Physic. Rev., 1931,38, 2131. 6 Ku, Physic. Rev., 1933, 44, 376. AF(638)-593), MU. 1963.

ISSN:0366-9033    

DOI:10.1039/DF9633500118

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Absorption Spectrum of Sulphur Dioxide in the Vacuum Ultra-violet BY I. DUBOIS* AND B. ROSEN Universit6 de Lihge, Institut d’Astrophysique, Cointe-Sclessin (Belgique) Received 18th February, 1963 A preliminary study of the absorption spectra of SOi6 and SO;* has confirmed that at least two transitions are involved in the 2300-1800 8, region and some new information has been gained concerning one of them. Further studies are needed to settle the possible geometrical asymmetry of the molecule in the upper electronic levels. INTRODUCTION Interest in the SO;! spectrum is mainly due to the possibility of a geometrical asymmetry of the molecule in some at least of the excited states. Such an asym- metry, due to a potential function with a double minimum in the antisymmetrical normal co-ordinate, has been tentatively assumed by Coon and Ortiz 1 and by Mulliken 2 to explain some peculiar intensity distributions observed in the spectrum. In fact, we 3 have been able to arrange the main bands in the far ultra-violet part of the spectrum in a vibrational scheme involving all three frequencies of the excited states in a way which would be contrary to the selection rules for a symmet- rical molecule.This arrangement was based on spectra obtained with moderate dispersion and cannot be considered conclusive. The difficulties of the analysis are even greater since it is not even known with certainty if the numerous strong bands in the 2300-17OOA region belong to a number of independent systems or form a single transition. The bands between 2300-1800 A have been arranged by Duchesne and Rosen 4 and Rosen 5 in four independent systems al, a2, a3 and a4 in general agreement with the theoretical expectation of Walsh.6 However, Riggs and Coon 7 admitted that all observed bands might belong to a single transition, in agreement with earlier assignments by Price and Simpson.8 In order to settle this question, we have re-investigated the bands of SOi6 and S 0 i 8 using spectra obtained in Dr.Douglas’s laboratory at the National Research Council in Ottawa in the 6th order of a 10-m concave grating, the dispersion being about 0.25I$/mm. We are much obliged to Dr. Douglas who kindly put these spectra at our disposal.? The spectra have been measured in Likge and this paper contains the preliminary results of the vibrational analysis.VIBRATIONAL ANALYSIS All bands measured in the absorption spectrum of SOi8 can be arranged in one system which is similar to the a2 system of SOi6 as proposed by Duchesne and * Aspirant of the Belgian National Foundation of Scientific Research. t We are also obliged to Dr. L. C. Leitch of the National Research Council in Ottawa for pre- paring a pure sample of SO\* for this experiment, as well as to Mr. F. Alberti for taking the plates. 124I . DUBOIS A N D B . ROSEN 125 Rosen. The heads and the corresponding vibrational quantum numbers are given in table 1, together with values calculated from the expression v = 42276 + 7455 + 379~; - 6&2 (1) The differences Vobs.-vcalc. are considerable and the constants in (1) are not reliable; but taking into consideration the complicated structure of the bands and the difficulties of a rotational analysis (even the K-structure being only partly resolved), the agreement is considered satisfactory.TABLE BAND HEADS IN THE SPECTRUM OF SO;* v ; v; v; obs. calc. v; v; v; obs. calc. 5 3 0 5 4 0 6 2 0 6 3 0 6 4 0 6 5 0 7 3 0 7 4 0 7 5 0 8 3 0 8 4 0 8 5 0 8 6 0 9 4 0 8 7 0 9 5 0 47 074 47 410 47 458 47 822 48 184 48 510 48 5675 48 9105 49 235 49 303 49 651 49 973 50 294 50 406 50 579 50 720 47 084 47 421 47 480 47 829 48 166 48 491 48 574 48 911 49 236 49 319 49 656 49 981 50 294 50 401 50 595 50 726 8 8 0 9 6 0 10 4 0 9 7 0 10 5 0 9 8 0 10 6 0 11 4 0 9 9 0 10 7 0 11 5 0 10 8 0 11 6 0 12 4 0 11 7 0 11 8 0 50 885.5 51 046.5 51 151 51 348 51 465.5 51 645 51 778-5 51 883.5 51 926.5 52 086 52 192 52 375 52 535 52 634.5 52 840 53 121 50 884 51 039 51 146 51 340 51 471 51 629 51 784 51 891 51 906 52 085 52 216 52 370 52 529 52 646 52 830 53 115 The proposed arrangement of the SO;* bands and their correlation with the corresponding SQk6 bands requires an adjustment of the earlier analysis of the system a2.The extrapolation of the observed isotopic shift indicates that the 0, 0 band lies for S 0 i 6 at about 42225 cm-1. All measured bands can be approxim- ately represented by the expression v = 42224 + 7554 + 422.5~; - 3.5~;2 (2) No deduction can be made from the change of the anharmonicity which might be due to the limited precision of this representation. DISCUSSION The spectra of SQi8 at our disposal do not include the region of the strongest a1 bands.The extrapolation using eqn. (2) leads, however, to conclusions con- cerning this system. In fact, the extrapolation includes all bands attributed by Duchesne and Rosen to a2 as well as all bands observed earlier by Chow 9 in the corresponding region but none of the bands which entered in the a1 system." It seems, therefore, that at least two systems are present in the 2100-19OOA region. We hope to extend the vibrational analysis to the region beyond 2100 and below 1900 A, in order to obtain further information about the other transitions * These latter bands have been used by Coon, De Wames and Loyd 10 to determine the geometrical configuration of the upper state. It seems, therefore, that the conclusion reached by these authors concerns the system MI, whose origin happened to lie near the origin of cc2.126 SPECTRUM OF SULPHUR DIOXIDE in SO2, and also to achieve a partial rotational analysis of the 1x2 bands. The only data obtained up to now concern the K structure. From the few bands in which this structure is clearly recognizable we obtain ( A ' - 2 ) = 0*90+0-02 cm-1. 1 Coon and Ortiz, J. Mol. Spectr., 1957, 1, 81. 2 Mulliken, Can. J. Chem., 1958, 36, 10. 3 Dubois and Rosen, Conference on Spectroscopy (Heidelberg, 1961). 4 Duchesne and Rosen, J. Chem. Physics, 1947, 15,63 I. 5 Rosen, J. Physique Rad., 1948, 9, 155. 6 Walsh, J. Chem. SOC., 1953, 2283. 7 Riggs and Coon, Symp. Mol. Structure and Spectroscopie (Columbus, June, 1958 ; AFOSR- 8 Price and Simpson, Proc. Roy. SOC. A, 1938, 165,272. 9 Chow, Physic. Rev., 1933, 44,638. TR-58-206, 1959). 10 Coon, De Wames and Loyd, J. Mol. Spectr., 1962, 8, 285.

ISSN:0366-9033    

DOI:10.1039/DF9633500124

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Rotational Analysis of Bands of the 3800 A System of SO, BY A. J. MERER Physical Chemistry Laboratory South Parks Road Oxford Received 14th January 1963 A partial rotational analysis of some bands of the 3800 8 absorption system of sulphur dioxide is described. The bands analyzed are perpendicular-type but of irregular structure each band displays a sharp secondary intensity peak 20 wave-numbers from the main maximum while the sub-bands on the violet side of each band are doubled. These irregularities are interpreted in terms of a triplet upper state 3B1 with transitions to two of the three components of a level of given N and K being resolved in the spectrum; for AK = +l the three brightest branches are RR31 R e 3 1 and R R ~ I (in this order for medium values of K ) .The three spin-splitting constants of a triplet state in a bent triatomic molecule are evaluated for the first time. A rotational perturbation presumably resulting from Coriolis-type interaction of the form B1-A2 appears in the 110-000 band. The longest wavelength absorption system in the electronic spectrum of sulphur dioxide consists of a number of extremely faint perpendicular bands between 3900 and 3400 A ; according to theoretical predictions 192 this transition should be 2blt4a1 but the assignment is not definite since the nearby strong transition with intensity maximum at 2900A has rotational structure too complicated to allow its polarization to be determined. This was clarified by Douglas,3 who showed that the 3800 A system has a pronounced Zeeman effect so that the upper state is probably a triplet state.(This follows directly because a large Zeeman effect results from the presence of electronic angular momentum and since for a bent triatomic molecule A is undefined any angular momentum must arise from electron spin.) On the other hand the 2900A system (which displays no Zeeman effect) has an extensive vibrational structure and therefore must have an upper-state geometry very different from that of the 3800 A system so that it is not possible to say with coddence that the two are related as singlet-singlet and triplet-singlet transitions. A provisional vibrational analysis 4 has fitted 30 bands of the transition into a scheme involving the three upper-state frequencies with magnitudes o.11 = 990.80, 0 2 = 385-1 5 and o.13 = 940.30 cm-1 and also indicates that the antisymmetric stretch-ing frequency appears more prominently in the spectrum than would normally be expected this has been discussed by Coon 5 and Mulliken,z who state that this fact, together with the large positive anharmonicity x33 is evidence for a slightly unsymmet-rical form of the molecule in its upper state in which one S-0 bond is longer than the other.Such a form will belong to point group Cs in which Av3 # 0 is allowed and is likely to have positive anharmonicity. A provisional rotational analysis,6 using rather lower resolution than the author gives constants for the 000-000 and 000-010 bands and mentions without explanation two features the first is that some of the sub-bands appear to be double-headed and the second is that a subsidiary intensity maximum 20 wavenumbers to the violet of the main maximum, occurs in the bands analyzed.The present investigation was carried out with a view to finding out from the rotational structure the exact nature of the upper state of the 3800A bands with particular emphasis on the presence of antisymmetric stretching frequency bands, following unfinished work on the spectra of CF;! and SiF2. 12 128 TRIPLET BANDS OF so2 EXPERIMENTAL Photographs of the 3800 8 and parts of the 2900 A systems of sulphur dioxide were taken in the third and fourth orders of the Oxford 21-ft. grating spectrograph using Ilford N 50 plates with a xenon arc as source of continuum the resolving power obtained is estimated to be close to 400,000 in the third order.Cylinder sulphur dioxide supplied by B.D.H. Ltd. was used without further purification. A path length of 6m at room temperature was used for the 38WA system with pressures from 6 to 25 CM Hg ; it was found that pressure broadening is very considerable above 20cm pressure so that the plates that were used for measurement were taken at 6 and 10 cm pressure. For the 2900 A system pressures of about 8 cm (in a 75 cm tube at -40°C) were used to photograph the region of the 030-400 band. RESULTS At fist sight the 3800A system of sulphur dioxide consists of four well-resolved and separated bands which seem to be straightforward perpendicular bands, followed by a large number of overlapping fainter bands of great complexity. In the four bright bands the sub-band structure degrades to the violet while to the red of the main maximum are large numbers of jumbled lines presumably P lines, extending for about 100 wave-numbers.The higher bands of the transition have very irregular K-structures with about half the spacing found in the four bright bands though one point common to all the bands is that the secondary sharp intensity peak about 20 wave-numbers to the violet of the main maximum always occurs. The first band to be investigated was the 0 0 0 ~ 0 band at 25750cm-1 and an analysis was performed as for an ordinary perpendicular band in the expectation that the analysis might be fairly straightforward and that the irregularities would be straightened out in the course of the analysis possibly as fortuitous blends of lines.The analysis was in fact very simple for in the sub-bands K" = 15-20 the only lines that could be seen were the lines of the RR branches and these immediately showed the overall pattern of the band-the polarization was perpendicular since had it been parallel it would not have been possible to observe heads in the Q branches for low K values such as are in fact found for the RR branches with K< 9. Nowhere in the spectrum were any effects attributable to the asymmetry of the molecule observed and the analysis was carried out assuming the molecule to be a symmetric top in both states. The only difficult part was the numbering of the sub-bands which also deter-mines the J-numbering automatically from the number of missing lines; this had to be done by calculating back to the main maximum of the band.Altering the K-numbering by one moved the calculated position of the main maximum by seven wavenumbers but rather surprisingly the best fit was for a half integral value of K ! The effect of the centrifugal distortion term D t was later found to be respon-sible for the difficulty in getting an unambiguous numbering since for K>7 the resulting diminution of the sub-band spacings becomes noticeable. The numbering was finally confirmed by the fmding of Q-branches in the sub-bands K" = 6-17, using as guide the ground-state combination differences calculated from micro-wave 7 and infra-red data.8 Doubts about the simplicity of the analysis arose when an attempt to calculate the complete band structure was made for the double heads of the sub-bands could not be fitted in either with blended lines or with the P-form sub-bands; in the latter case no heads would be expected since the K-numbering of the pR branches would have to be too high to allow head-formation and also the intense branches of the P-form sub-bands would have been pp rather than PR.The fainter double A. J. MERER 129 components of the sub-bands could be seen to consist of one branch in each sub-band only starting 2-3 wave-numbers to the red of the main RR branch and notice-ably less intense than it. Two possibilities could account for this doubling: (i) That the transition is a singlet-triplet intercombination. This would account for Douglas’ observation of a Zeeman effect and for the faintness of the bands but not fit in with the different geometry expected for the nearby corresponding singlet-singlet transition nor account for the fact that the bands are doubled not tripled.(ii) That the transition involves an unsymmetrical upper state with barrier to inversion sufficiently large to cause inversion doubling. This could account for doubled bands possibly with one component brighter than the other but not for the subsidiary maxima unless they are pR heads. Either explanation requires a reason for the appearance of vibronic-forbidden A2 bands either as the antisymmetric components of the inversion doubling or the antisymmetric stretching frequency bands. Analogy with the spectrum of formaldehyde was the deciding factor in the interpretation. Formaldehyde has two absorption systems in the near ultra-violet, 3A2 (3A”)+lAl and 1A2 (1A”)+-lA1 the singlet system appears in absorption as a result of excitation of non-totally symmetric vibrations while the triplet system ap-pears as a result of spin-orbit interaction 9 with a higher excited 1Al state.If the 3800A system of SO2 were to show inversion doubling then the antisymmetric components of the doubling would have vibronic symmetry 1A2 which would be forbidden in the absorption spectrum except as a result of possible rotational-electronic interaction or an allowed magnetic dipole transition 9 (in which case they would appear as parallel bands) if this was so and the minus components appeared as parallel bands with their heads causing the subsidiary maxima then the bands should not be doubled.More likely was that the 3800A system represents the transition 3B1-1A1 with the vibronic forbidden antisymmetric stretching frequency bands allowed as a result of spin-orbit interaction. Further internal evidence was against the inversion theory the provisional vibrational analysis 4 showed that since the vibration v3 has positive anharmonicity no doubling would be expected (see fig. 11 while the centrifugal stretching terms calculated for the less intense components on an inversion model are so different from those calculated for the more intense components that the two cannot be related. Barrier to None inversion Small Large FIG. 1 .-Shapes of the potential curves for the antisymmetric stretching vibration of a bent triatomic molecule for zero small and large barriers to inversion.SINGLET-TRIPLET BANDS Since the anomalies in the rotational structure of this transition could not be explained except in terms of a singlet-triplet transition the next steps were to decide what a perpendicular singlet-triplet transition should look like and to evaluate the spin-splitting constants. 130 TRIPLET BANDS OF so2 Three branches only appear in the violet half of each band two RR branches, one brighter than the other and one branch going with the brighter RR branch. Presumably the branch corresponding to the fainter RR branch and the two RP branches are also present but too faint to be picked out. Attempts to find a third RR branch were not successful for although some of the sub-bands of the 010-000 band look as though there is a third head just to the red of the two RR heads this can be explained as a blend of an R and a Q line so that its existence is doubtful especially as it only occurs in one band.Formulae for the energy levels of a triplet state of a non-linear molecule have been given by Henderson and Van Vleck,lo and more accurately by Henderson.11 These formulae are given as perturbation terms to be added to the regular energy levels of an asymmetric top and consist of two parts a “ dipole term ” correspond-ing to the term in y for diatomic molecules and a “ pseudo-quadrupole term ”, corresponding to the term in A. Henderson has reduced the dipole term of his formula for the asymmetric top to a form applicable to a symmetric top and for a level of given N and K obtains where A and ,u are constants depending on the structure of the molecule and the molecular coupling constants and C is given by Henderson has not reduced the pseudo-quadrupole term but making the approx-imation that Henderson’s constant ( P - y ) becomes zero as the asymmetry para-meter b tends to zero for a symmetric top the pseudo-quadrupole term becomes C = J(J + 1) - S(S + 1) - N(N + 1).where a and p are interaction terms arising from the presence of nearby states along the axes corresponding to the rotational constants A and z A‘ is the spin-orbit coupling constant and 5 is a complicated function of J S and N : cl u - $C(C+l)-S(S+l)N(N+l) u- (2N - 1)(2N + 3) More simply this can be written WP - N ( N + l ) 3 where A’ = 4(A’h)2(a-j).This result differs slightly from that deduced by DiGiorgio and Robinson,l2 in their analysis of the 3A2-1A1 bands of formalde-hyde in the inclusion of the term in K2/N(N+ 1). For a triplet state where S = 1 C and 5 can be evaluated - F1 levels J = N f l C = 2N E = (N2-$N)/(2N- 1)(2N+ 3), F2 N -2 (1 4 - 2N- 2N2)/(2N - 1)(2N + 3), F3 N- 1 -2N-2 (N2+2$N+ 1&)/(2N- 1)(2N+ 3), and when substituted into the full formula, they give the required energy levels. triplet state of a non-linear polyatomic molecule.11 Case (b) coupling is assumed to hold for PLATE 1 .-Resolvable structure on the violet side of the OOo-OOO band several R-form sub-bands PLATE 2 . 4 4 The 010-000 band showing the two maxima and some of the R-form sub-(6) Region round the secondary maximum in the 000-000 band.(c) Perturbed region A . J . MERER 131 At this stage what are needed are intensity predictions similar to those of Honl and London 13 for singlet-singlet transitions but giving the line-strengths for a singlet-triplet transition. Failing these it is impossible to be certain of which branches are actually observed in the spectrum and it was necessary to fall back on analogies and trial-and-error methods. INTERPRETATIONS OF THE SPECTRUM The structures of the diatomic transitions 3C+- lZ+ and A - l I T show that for an intercombination of multiplicities the branches where AN# AJ become strong in a triplet-singlet transition (and in the former the branch is stronger than the R branch). Then by analogy for any perpendicular triplet-singlet transi-tion in a symmetric top molecule the branches with AN# AJ probably will be of comparable intensity to those with AN = A J since J is the important quantum number governing the branches formed rather than N one can expect a transition of this type to have the nine P-form and nine R-form branches illustrated in the energy level diagram (fig.2). Q-form branches would not be expected since Hund's K 5 6 7 F3 F2 F1 F3 F2 F F3 F2 El J - J N - J N - N 9 - 8910 9 - 89'o 9 - 89'o 7 ~~ Iji a K I' 6 'A, U FIG. 2.-Energy level diagram for the sub-band K"=6 of the transition 3B1-1A1 the branches resolved are RR31 R e 3 1 and RR21. Only the first line of each branch is shown. case (b) applies and the important quantum number for axial angular momentum will be K rather than P.14 Consideration of this energy level diagram shows tha 132 TRIPLET BANDS OF so2 the splittings appearing in the spectrum do not represent the actual splittings of the upper state levels of given N since different lower state levels are involved.Assuming that the two R-branches resolved in the spectrum are RR31 and RR21 the " true " splittings are obtained by adding the appropriate ground state combination differences : F,(N,K'= K"+1)-F3(N,Kf = K"+1)= RR21(J= N-l,K")-RQ31(J= N-1,K") = RR2,(J = N - 1 K")-RR31(J = N-2,K") +AlF"(J = N - 2 K"). It is equally possible that the R branches seen might be r;i and Fz or F1 and F3, and the corresponding splittings become Fl(N,K' = K"+l)-F2(N,.K' = K"+l) = RRl,(J = N,K")-RQ,l(J = N,K") = RR,l(J = N K")-RR21(J = N - 1 K") +AlF"(J = N - l K " ) , F l ( N K' = K" + 1) - F3(N K' = Kf + 1) = RRll(J = N K")- "Q3i(J = N - 1 K") = RRll(J = N K")-RR,l(J = N - 2 K") +A1F(J = N - 1 K") + A2Fr(J = N - 1 K").The first lines of the RRll branches have to be RR11(J = K+ 1 K ) so that if the brighter component is F1 the apparent K-numbering obtained from the combin-ation differences between the R- and @branches must be changed by one though the observed splittings without the AIF" term added can be used. Since Henderson states 11 that the splitting caused by the dipole term would be small and at most only about Ncm-1 it seemed that there were only three possible ways of pairing the RR branches to give a splitting of this magnitude (fig. 3). Pairing violet a o FIG.3.-Dossible pairing schemes for the sub-band components of the spin-splitting. The sub-band heads filled in with black are those for which the quantum numbers are definitely known from combination differences. (1) gives a likely range of splitting extrapolating nearly to zero at N=O and K=O, but varying considerably (some 20 %) with vibrational quantum number 212 though not so much with v1. Pairing (2) is the least likely as it gives the largest splitting, and the largest variation with u while pairing (3) is possibly more likely than pairing (1) as it gives a small variation with v (only 1% % between the three bands analyzed), although the splittings are larger than those of pairing (l) not so sensitive to changes of Kz and do not extrapolate to zero.Since the effect of vibration on the spin-splitting constants should be small if the case of Sz is any guide,ls pairing (2) can be discounted while it would be remarkable if the variations in pairing (3) were co-incidence. One may note that from Henderson's equations the splitting might be erratic at low Nand K A . J . MERER 133 In the absence of intensity predictions indicating which two of the three R branches should appear in the spectrum and of evidence about the variation of spin-splitting with 01 and 02 the only way to continue was to fit the splittings cal-culated for the three bands analyzed for pairings (1) and (3) against the theoretical splittings F1- F2 F1- F3 and F2 - F3 calculated from Henderson's equations. This was done by means of multiple regressions in A p and A' programmed for the Oxford University Ferranti Mercury computer.The expected splittings are : and the results of the multiple regressions are given in table 1. F3-F1 and F3-F2 are not given in the table they were tried for the 000-000 band and gave constant terms of about -20 cm-1. Since these are impossible they have been disregarded. Table 1 shows that pairing (1) gives the lower constant terms but in neither case is it possible to decide between the alternatives. TABLE 1 .-RESULTS OF MULTIPLE REGRESSIONS I p (3) 1' const. I P const. (l) 1' pairing F2-F3 000 0.0442 0.310 1.01 6.06 0.0959 -0.215 -0.276 1.50 010 0.0385 0.296 1.27 6.00 0.112 -0.222 -0.274 2.57 100 0.0308 0.325 0.598 5.63 0.112 -0.180 1.61 2.39 Fi-F3 000 0.0162 0.340 8.83 4.54 0.0501 -0.108 -1.70 1.82 010 0.0219 0.341 11.57 4.28 0.0575 -0.112 -2.25 2.92 100 0.0126 0.339 6.04 4.46 0.0459 -0.095 11.7 2.66 F 4 7 2 ooo] 010 not tried loo] 0.106 0.0652 -0.432 1-35 0.147 0.0565 -0.856 2.09 0.123 0.106 -2.34 2.41 F2-Fl 000 -0.0635 0.0741 1.48 4.89 -0.104 -0.0637 0.431 1.29 010 -0.103 0.0707 2.61 4.47 -0.144 -0.0551 0.894 1.95 100 -0.0582 0.0872 0.915 4.30 -0.122 -0.106 2-47 2-47 All values are in cm-1.However the interpretation of the subsidiary maxima still remains since they must be associated with the triplet upper state of the transition it is only possible to assign them to the P-form structure of the bands. Again since they always appear to the violet of the RR(K=l) head there are only two interpretations both assuming pairing (3) the first is that they represent the K' = 1 +K" = 2 head, with F2 - F1 (not F1- 8'2) appearing in the R-form sub-bands and the second is that they represent the K' = OtK" = 1 head with F2-F3 on the violet side.Assuming pairing (1) it is not possible to explain them while pairing (1) is definitely inferior to pairing (3) in the interpretation of the perturbations in the 110400 band so it seems that pairing (3) is the correct one 134 TRIPLET BANDS OF so2 The Honl-London intensity formulae show that there is no intensity minimum at the centre of a perpendicular band i.e. that the sub-band K’ = OtK” = 1 should be just as intense as the sub-band K’ = I c K ” = 2 neglecting Boltzmann factors etc. thus it is not possible to assign the subsidiary maxima to the sub-bands K’ = I t K ” = 2 unless some feature corresponding to the K’ = OtK” = 1 sub-bands can be found.Since no such feature appears this rules out the first interpretation in the last paragraph and shows that the intense branches are RR31 and RR21 with RR31 the brighter. The correct spin-splitting constants are thus those labelled Fz - F3 in pairing (3). This assignment seems to run contrary to the results of the multiple regressions, and it can only be assumed that the line accuracy of the I72 levels is low because of their faintness and the severe blending or that the F2 assignments are wrong in some of the sub-bands where the R-branches of the F2 levels form heads. The spin-splitting constants are not sufficiently accurate to justify including them in the final structural constants of the molecule.The latter were determined assuming the F3 levels (the brighter components) to behave like the levels of a near-prolate symmetric top in a singlet state (table 2). Terms in DJ and HK have been neglected, though H i must be appreciable. The value of DiK is unreal as it contains a large spin-splitting contribution. TABLE 2.-ROTATIONAL CONSTANTS FOR THE 3B1 STATE OF $ 0 2 * - -level vo A’-B’ D k B’ %K I v 1‘ 000 25766.88 2.0161 0.000212 0.2799 - 8.2 x 10-6 0.044 0.31 1.0 010 26126.73 2.1030 0.000265 0.2796 -77.7 0.039 0.30 1.3 100 26672.43 2.0055 0-000226 0.2797 -6-5 0.031 0.33 0-6 110 27026.45 2.0645 0.000275 0.2796 -4.8 - - -* assuming infra-red 8 16 and microwave 7 constants for the ground state. For the 000 level of the 3B1 state the structural constants (assuming equal S-0 bond lengths) are rs-0 = 1.4944 A: 0 = 126” 04;.PERTURBATIONS I N THE LEVEL 110 Two distinct perturbations appear in the 110-000 band. One is centred on the upper state levels K’ = 12 and its effect on the brighter components is to disrupt the sub-band K” = 11 completely after the first two R lines and to alter the B;;“ values of the nearby sub-bands K” = 9 and 10 have their lines displaced to the red while K” = 12 and 13 are similarly displaced to the violet; their sub-band origins are not however affected. K” = 11 may revert to its unperturbed condition after J = 17 but blending is severe and confirmation from @branches is lacking. Apparently the perturbation in the fainter sub-band components principally affects the sub-band doubling K” = 12 this is an important point because if the com-ponents are perturbed at the same K value (a plausible assumption) then the component-pairing occurring in the spectrum is pairing (3).The second perturbation is of a different type and disrupts all the observed levels from K’ = 15 upwards. The appearance of the spectrum is little groups of five Gr six regularly spaced lines at irregular intervals and no definite quantum numbers can be assigned since no confirming Q lines can be found. The perturbation seems to be centred between K’ = 16 and K’ = 17 and may only extend from K’ = 15 to K‘ = 19. As for the cause of these perturbations Prof. Coon in a private communication has suggested that they occur because a level involving v3 lies close to the level 110 A.J . MERER 135 and interacts with it. Coriolis interaction of this type between an A2 and a B1 vibronic state is the simplest explanation of the perturbation though it seems that there are actually two perturbing influences operating. If it is a Coriolis-type per-t urbation that affects the sub-band K" = 11 then the level responsible must have an origin above that of the 110 level and rotational constant A - less than that of the 110 level. From Russell Landrum and Vezey's vibrational analysis 4 one can calculate the levels 100 001 110 and 01 1 to lie at 904 913 1258 and 1240 cm-1 above level 090 however no perturbation appears in level 100 although this might be expected to suffer considerably from interaction with level 001 and the most likely explanation is that the constants derived for v3 must be slightly in error so that 01 1 lies slightly above 1 10 and 001 rather further above 100.The perturbations are illustrated in plate 2. The sub-band head K" = 2 seems to be missing so there might be a third per-turbation practically at the origin. DISCUSSION In this analysis of the brightest bands of the 3800A system of S 0 2 the upper state has been shown to be a triplet state in which the molecule has S-0 bond lengths (assuming them equal) 1.4944& and 0-S-0 angle 126" 5' constants for the triplet splitting have been derived. An energy level diagram for the transition 3B1-1A1 is given and for AK = + 1 transitions to the upper state F3 levels are more intense than those to the F2 levels while no transitions to F1 levels have been seen.The important point that emerges is that the corresponding singlet state has very different geometry this fact has so far hindered a complete understanding of the SO2 spectrum and it is possible that a similar situation might occur in other molecules of this type. Many problems have been raised in the course of this work especially concerning the vibration v3. Several levels with vibronic symmetry 3A2 are alleged to occur in the spectrum4 (for instance the strong band at 28330 cm-1 is given as 201-000) and one must ask why this band should be nearly as strong as the four bright 3B1 bands and again why only some of the antisymmetric levels occur in the spectrum. The answer to the first is probably that the spin-orbit interaction mixing the 3Az levels with higher singlet levels and hence allowing them to appear in absorption is nearly as strong as the interaction which allows the 3B1 levels to appear; since the 3A2 levels appear also as perpendicular bands, it can be assumed that they have been mixed with a 1B2 state,g while the 3B1 levels have been mixed with a 1Al state.The second remains unanswered. The large change of geometry between the 1B1 and the 3B1 states is perhaps not so inexplicable when the molecular orbitals are considered : (,J4(7d2. '2; ( l a d2 (3b2 >2 (4a 1) ' A ground linear bent (4al)'(2b1)' 3B1 3800 A (4a1)'(2b1)' ' B 2900 A (4a1)0(2b1)2 ' A 2 electron jump (n,)3(7L>3 'Z (la2)'(3b2)2(4a,)2(2b,)' ' B 2350 A etc 136 TRIPLE BANDS OF so2 The 3B1 state correlates in the linear molecule with 3Z; and the 1B1 state with l A g ; even so the change of geometry would not be expected to be so large since the states have the same electron configuration.The author would like to express his thanks to Dr. R. F. Barrow under whose guidance this work was done for discussions on the structures of triplet states; to Prof. C. A. Coulson F.R.S. for clarifying the double minimum problem; and to Prof. J. B. Coon and Dr. R. K. Russell of A. and M. College Texas for ex-tremely valuable criticism and help. The author would also thank the Warden and Fellows of New College Oxford for the award of a Stone-Platt Studentship and the D.S.I.R. for a maintenance grant. 1 Walsh .I. Chem. Soc. 1953 2283. 2 Mulliken Can. J. Chem. 1958 36 10. 3 Douglas Can. J. Physics 1958 36 147. 4 Russell Landrum and Vezey ASTIA doc. AD 81060 (1956). 5 Coon Bull. Amer. Physic. Soc. 1957 2 100. 6 Russell AST'A doc. 152030 (1958). 7 Sirvetz J. Chem. Physics 1951 19 938. 8 Shelton NieIsen and Fletcher J . Chem. Physics 1953 21 2178. 9 Sidman J. Chem. Physics 1958 29 64.4. 10 Henderson and Van Vleck Physic. Reu. 1948 74 106. 11 Henderson Physic. Reo. 1955 100 723. 12 DiGiorgio and Robinson J. Chem. Physics 1959 31 1678. 13 Honl and London Z. Physik 1925 33 803. 14 Mulliken J. Chem. Physics 1955 23 1997. 15 Barrow and Ketteringham Can. J. Physics 1963 in press. 16Danti and Lord J. Chem. Physics 1959 30 1310

ISSN:0366-9033    

DOI:10.1039/DF9633500127

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Absorption Spectrum of Chlorine Dioxide in the Vacuum Ultra-violet BY C. M. HUMPHRIES, A. D. WALSH AND P. A. WARSOP Chemistry Dept., Queen's College, Dundee Received 15th January, 1963 The far ultra-violet absorption spectrum of C102, previously thought to be continuous, has been found to contain three band systems, at 1829, 1628 and 1568 A. Reasons are given why these new systems are ascribed to C102 itself and not, e.g., to a photolysis product. Each of the systems represents a Rydberg transition. The 1829 and 1628 8, systems are associated with the first ioniz- ation potential, the 1568 8, system with the second ionization potential. The upper states of the first two systems (and the ground state of the ClOi ion) have shorter C1-0 bond lengths than the ground state of C102, but OClO angles not greatly changed.The upper state of the third transition (and the first excited state of the ClO; ion) have OClO angles considerably increased from the C102 ground state value, but CI-0 lengths probably not greatly changed. The symmetrical stretching frequency v l is increased in the 1829 and 1628 A transitions from its ground state value; the bending frequency v2 is increased in the 1628 and 1568 8, transitions from its ground state value. The 1568 8, transition appears as a progression of bands with alternate long, short spacing; Fermi resonance between v; and 2 4 may account for this. The absorption spectrum of the C102 molecule in the near ultra-violet has been extensively studied and discussed by Coon and co-workers.l-3 The spectrum in the vacuum ultra-violet has previously been thought to be entirely continuous,4 but we have now discovered three new electronic systems with vibrational structure. EXPERIMENTAL Chlorine dioxide was prepared by heating moist potassium chlorate with oxalic acid, according to directions given by Brays and by Coon.1 After condensing the gaseous products in a trap at -8O"C, C02 was pumped off and the C102 purified by distillation in vacuum.The absorption spectrum was obtained with both a 1 m vacuum spectrograph giving a path length of 2 ni and a dispersion of CCL. 16 &mm and with a 2 ni vacuum spectrograph giving a path length of 4 m and a dispersion of ca. 8.7 A/mm. RESULTS The absorption spectrum has been photographed from ca. 3300 to 1lOOA. At the lowest pressures used, continuous absorption is evident at the shortest wave- lengths.As the pressure is increased, this continuous absorption gradually spreads to long wavelengths until, at the highest pressure used, there is a short wavelength cut-off at about 2000A. At intermediate pressures, three sets of bands are visible. Over the same range of pressure, the bands of the near ultra-violet system become strong on our plates and eventually, at the highest pressure used, spread (from a maximum at ca. 3300A) to about 2700A. The two shorter wavelength sets of far ultra-violet bands have about the same appearance pressure, this being slightly less than that for the longest wavelength set and comparable with that for the strongest near ultra-violet bands. The bands of each set increase in intensity with pressure, though apparently more slowly than do the bands of the near ultra-violet 137138 SPECTRUM OF CHLORINE DIOXIDE system.With grosser increments of pressure than used here, because of the con- tinuous absorption spreading to long wavelengths with increasing pressure it would be easy to miss the far ultra-violet band systems. The first system begins at 1829 A and consists of three bands whose intensity falls off rapidly from the first to the second to the third. These bands (A, B and C) A B FIG. 1.-(a) The 1829 8, bands; (b) the 1628 and 1568 8, systems. The intensity scale is not the same for (a) and (b). occur at 54,689, 55,709 and ca. 56,720 cm-1 respectively, the separations being 1020 and ca. 1011 cm-1. The system is fragmentary because of the decreasing intensity of the bands towards shorter wavelengths and because of the cut-off due to the continuous absorption.A densitometer tracing of the bands is shown in fig. 1.C. M. HUMPHRIES, A. D. WALSH A N D P . A . WARSOP 139 The second band system begins at 1628 A. The bands (D E F G H J K') are shown in the densitometer trace reproduced in fig. 1 and their measurements are given in table 1. Two frequency separations are involved. The main bands (D F H K') form a progression with spacings of 1051, 1091 and 1068 cm-1. Associ- ated with each main band lies a weaker band ca. 520 cm-1 to the violet. Again the first band is the strongest and the intensity falls off towards shorter wavelengths where the system is overlapped by the third set of bands.TABLE FREQUENCIES OF THE BANDS IN THE 1628 A SYSTEM label cm-1 D 61y430-> 521 \>,,,, E 61,951- F G 63,007 - H 63,572--- 62,48 1 ---- / > 526 '\ J ca. 64,087 K' 64,640 The third band system begins at 1568 A. Its bands (I K L M N 0 P Q) are shown in the densitometer trace reproduced in fig. 1 and their measurements are given in table 2. The bands are somewhat sharper than in the other two systems. TABLE 2.-FREQUENCIES OF THE BANDS IN THE 1568 8, SYSTEM label I K L M N 0 P Q cm-1 (4 (b) 63,774 - 64,282 64,778 65,281 65,770 - 66,28 1 66,767 ca. 67,280 - The apparent intensity of band K may be lowered by the presence of an emission line on its short wavelength side ; if so, the intensity of the bands may vary smoothly with wavelength and the bands might be regarded as forming a single progression in CU.500cm-1. Arranged as a single progression, the band separations are not constant but alternate long, short, long . . . (see column (a) of table 2). Alter- natively, the bands might be arranged as a progression I, L, N, P, each with an associated band (K M 0 Q respectively; see column (b) of table 2). The main1 40 SPECTRUM OF CHLORINE DIOXIDE progression spacing is then ca. 1000 cm-1 and the subsidiary spacing cd. 500 cm-1. If this arrangement is adopted, it should be noted that bands K M 0 Q are only slightly weaker than bands 1, L, N, P respectively, and therefore the arrangement presumably conceals progressions in ca. 500 cm-1. In other words, whichever arrangement is adopted, a progression in cu.500cm-1 is present. With either arrangement, maximum intensity is reached at band L. DISCUSSION For the following reasons the three new band systems observed are believed to be due to C102 and not to an impurity or product of photolysis. (i) The systems do not correspond to any of the known band systems of C12, 02, ClO, HC1, CO, C02 and H2O. The absorption spectra of all but the third of these have been well explored in the relevant wavelength region. (ii) The second band system involves two upper-state vibrational frequencies and must therefore be due to an absorbing species that contains at least three atoms/molecule. In view of this and of (i), CIOz seems the only likely absorbing species. (iii) The band systems have a very low appearance pressure and any impurity, present in small amounts, would have to be an extraordinarily strong absorber to produce the observed systems. (iv) The three band systems are all observed on plates that also show the known bands of C102 in the near ultra-violet.C102 molecules must therefore have been present in the spectrograph in considerable concentration at the time of photographing the new band systems. Photolysis can hardly have been very extensive. In any case, photolysis could only affect molecules in the light beam (whose volume is small compared with the total volume of the spectrograph) and diffusion out of the beam would tend to keep the concentration of products small. (v) The band systems increase in intensity as more C102 is added to the spectrograph. Assignment of the systems to a photolysis product would therefore only be plausible if the product responsible were sufficiently inert to survive the time (a few minutes) from one exposure to the next.(vi) The band systems were observed with two different samples of C102. (vii) If the far ultra-violet band systems are not due to C102, it is surprising that C102 should show no absorption as strong as the strongest bands of the near ultra-violet system until quite short wavelengths (5 1400 A) are reached. According to Dibeler, Reese and Mann 6 the first ionization potential of C102 is 11-1 eV. We should certainly expect strong Rydberg transitions between 2000 and 1400 A. Moreover, we expect these Rydberg transitions (which partially remove an electron from an antibonding orbital) to be discrete, since even the near ultra- violet transition (where an electron is transferred from a non-bonding orbital to an anti-bonding one) is discrete.In addition, the magnitudes of the vibrational frequencies involved in the three electronic systems are all plausible for C102 as the absorbing species; and may be linked, as we show below, with the observed high intensities of the transitions. The ground state fundamental frequencies of C102 are known 7 to be 943 cm-1 (vial, symmetrical stretching), 1 1 10.5 cm-1 (v&, asymmetrical stretching) and 445 cm-1 (vzal, bending). 1020, 1011 cm-1 in the first system and 1051, 1091, 1068 cm-1 in the second system only plausibly represent, therefore, the symmetrical stretching frequencies of the upper states.They are increased from the ground state value. Now any intra-valency shell transition of C102 is expected to increase the Cl-0 length and reduce v1 (see Walsh 8). Decrease of v1 on electronic excitation is indeed observed in the near ultra-violet band system which is undoubtedly an intra-valency shell transition.8 At least the first two of the three new electronic transitions must therefore be Rydberg transitions. The wavelengths at which the transitions occurC. M. HUMPHRIES, A . D. WALSH AND P . A . WARSOP 141 accord with this conclusion. In a Rydberg transition associated with the first ion- ization potential, an electron is removed from the anti-bonding bl -EU molecular orbital and placed in a large Rydberg orbital wherein it is expected to be without considerable effect on the dimensions of the molecule.v1 should therefore be in- creased by the high energy excitation, as observed in the first and second band systems. The conclusion that these two systems are Rydberg may be linked with their high intensities, since the early Rydberg transitions are expected to be of high intensity. Emax. for the near ultra-violet system is reported to be at least 2000.9 It follows from the appearance pressure data that the far ultra-violet systems also have Emax. of at least the same order of magnitude. Rydberg transitions have Emax. values up to at least 104. That v; is not very different in the first and second systems is expected because the upper states of Rydberg transitions leading to the first ionization potential should each approximate to the ground state of the ion ClO;.vi is, in fact, some- what greater for the second than for the first system and so the Cl-0 length is presumably somewhat less in the upper state of the second system than in the upper state of the first. That the bands of the main progression in the second system fall off in intensity more slowly than do the bands of the first system accords with v; -v’i being greater for the second system than for the first. Indeed, the association of the slower falling off in intensity with the higher value of v; constitutes in itself an argument that both band systems are due to a molecule whose v‘; frequency is less than the v j frequencies of ca. 1020 and ca. 1050 cm-1; and so there is a further argument consistent with C102 being the absorbing species.The frequency of ca. 520cm-1 in the second system must represent vi. The weakness of bands E, G and J relative to bands D, IF and H in the second system, and the non-appearance of bands showing a separation of ca. 500cm-1 in the first system, then implies that in both these systems the electronic excitation causes a much more important change in the Cl-0 distance than in the OClO angle. This is in accord with the diagram plotted by one of us 8 correlating the molecular orbitals of linear and bent C102; an electron in the bl-EB orbital has only a minor effect on the OClO angle. Accepting the first two band systems as the first allowed Rydberg transitions, the upper orbital of the 1829 A transition may be labelled (xul), while the upper orbital of the 1628 A transition may be labelled (pal) or (pbl).All the bands should therefore be perpendicular in type, in contrast to the parallel bands of the near ultra-violet system. The far ultra-violet bands are observed to be narrower than the near ultra-violet bands. The third new band system has about the same intensity as the first two transi- tions and is therefore probably a Rydberg transition also. The frequency of ca. 500 cm-1 must represent vi. On the other hand, the third system differs from the first two systems in that (a) the first band is not the strongest, (b) a considerably larger number of bands apparently forming a progression in vi is present, (c) the intensity of the (OlO)+(OOO) band is far higher relative to that of the (OOO)+(OOO) band.If the arrangement shown in column (a) of table 2 is adopted, v[ is apparently not excited and a considerable progression in v;, reaching maximum intensity in the third band, is present. If the arrangement shown in column (b) of table 2 is adopted, the frequency of ca. 1000 cm-1 must represent vi. In that case, v i is less in the third system than in either of the first two systems and any progression in v; would be expected to fall off in intensity even more rapidly than in the first system. Since, in fact, band L is more intense than band I, L cannot be simply the (lOO)+-000) transition, but must involve a considerable contribution from the (020) +(OW)142 SPECTRUM OF CHLORINE DIOXIDE transition. Similarly, comparison of fig. l(a) and l(b) suggests that band N must largely represent the (04O)t(OOO) vibronic transition of the third system.In cther words, we confirm the conclusion already reached that, however arranged, the bands of the third system involve a progression in v;. In contrast, v i does not appear in the first system and is only represented by very weak bands in the second system. From the vi considerations, the change in Cl-0 length brought about by the elec- tronic excitation is less in the third system than in the first and second systems; while, from the vi considerations, the change in OClO angle brought about by the excitation is much more profound in the third than in the first two systems. It is difficult, therefore, to suppose that the third system represents a Rydberg transition associated, like the first two systems, with the first ionization potential.The transition, however, does have the characteristics that might be expected for a Rydberg transition involving excitation of an electron from the a1 -FU orbital, i.e., a Rydberg transition associated with the second ionization potential. The nu orbital is C1--0 antibonding; but, by bending, the antibonding nature of the a1 component of %a is partially relieved and, especially at angles not too far removed from the ground state value of 116.5*,* the a1 -Zu orbital is expected to be largely a lone pair or non-bonding orbital.8 Removal of an electron from the al-n, orbital is ex- pected to cause a considerable increase in OClO angle (and hence a long progressiofi in the upper state bending frequency), but only a small change in C1-0 length (and hence little or no excitation of v;).Since the first excited state of the ion ClO$ differs from the ground state of NO2 by having an electron in the bl--Ku orbital, it seems probable that the OClO angle in the upper state of the third new band system is a few degrees less than the value (134") of the ON0 angle in the ground state of N O 2 . Assignment of the third new band system as the first Rydberg transition leading to the second ionization potential implies that (i) the upper orbital of the 1568 A transition is (sal) in type and the bands of the system are perpendicular; (ii) the upper orbitals of the 1568 and 1829 A transitions are indeed the same and, therefore, that (iii) the C102 molecule should possess an intra-valency shell transition, formulated as whose maximum intensity should lie at roughly the separation of the positions of maximum intensity of the 1829 and 1568 A transitions, viz., at ca.64,478 - 54,689 = 10,089 cm-1, which corresponds to ca. 10,000 A. A search for an absorption system in this difficult region of the infra-red may therefore provide a test of the assignments made here. The bands of transition (1) should be perpendicular in type and the transition should cause increases in both apex angle and C1-0 length. The near ultra-violet bands are satisfactorily assigned to the transition - * (a1)(b1)2, 2AlC. - @1)2(bl), 2B1, (1) * (a2)(b2)2(a1)2(b1)2, 2f42+. ' - (a2)2(b2>2(a1)2(b1>, 2B1. (2) Transitions (1) and (2) are the only allowed, low-lying transitions expected to possess discrete structure. Transitions involving transfer of an electron to the 21 -Zg orbital may well be responsible for the continuous absorption observed.It remains to discuss the alternation in spacing observed for the bands of the third far ultra-violet system. A possible explanation of this is that it is due to Fermi resonance. One would expect, from our assignment of the third system, that vi would be somewhat greater than v'i though less than v{ in the first and second systems. * from electron diffraction data.10 A combination of electron diffraction and infra-red data leads to the value 118.5".C. M. NUMPHRIES, A. D. WALSH AND P. A. WARSOP 143 v { may well therefore be - 1000 cm-1 as shown in arrangement (b) of table 2 ; and since v$ is -500 cm-1, Fermi resonance between vi and 2v5 is possible.The bands seem best regarded as primarily a v i progression ; but with some excitation of v i , because of Fermi resonance, in all bands from the third onwards. Data on the ground state levels of C02 (where v1 is well known to be in Fermi resonance with 2v2) show that it might be possible, because of Fermi resonance, for a vibrational progression to show an apparently alternating spacing. Thus, taking the highest energy level of each Fermi resonance group of levels for C02 11 an alternating spacing is evident : cm-1 0 667 1388 2077 2797 3502 4224 )667 )721 )689 )720 )705 )722 In a similar way, if particular members of each Fermi polyad are the most intense and the others are not resolved or are too weak to be observed, it might be possible to explain the alternating spacing found for C102.Reversing the argument, if Fermi resonance is accepted as the explanation of the alternating spacing, then v1 is to some extent increased by the electronic excitation of the third band system and we have a further argument that the third system is Rydberg in type. Fermi resonance may also play a part in determining the spacings of the bands in the 1628 system. The spacing between bands F and M is definitely greater than that between bands D and F, in spite of the expectation that in a v1 progression the spacing should decrease, rather than increase, as successive quanta of v1 are added. Further, ca. 520 is not far from ca. 1050/2 and there appears to be a short, long, short alternation in the spacings of bands D F H K’. It may be that the bands of the 1628 A system are best regarded as primarily a v; progression ; but with some excitation, due to Fermi resonance, of v$. 1 Coon, J. Chem. Physics, 1946, 14, 665. 2 Coon, Physic. Rev., 1952, 85, 746. 3 Coon and Qrtiz, J. Mol. Spectr., 1957, 1, 81. 4 Price and Simpson, Trans. Faraday Soc., 1941, 37, 106. 5 Bray, 2. physik. Chem., 1906, 54, 575. 6 Dibeler, Reese and Mann, J. Chem. Physics, 1957, 27, 176. 7 Nielsen and Woltz, J. Chem. Physics, 1952, 20, 1878. 8 Walsh, J. Chem. Soc., 1953, 2266. 9 Goodeve and Stein, Trans. Faraday Soc., 1929, 25, 738. 10 Dunitz and Hedberg, J. Amer. Chem. Soc., 1950, 72, 3108. 11 Taylor, Benedict and Strong, J. Chem. Physics, 1952, 20, 1884.

ISSN:0366-9033    

DOI:10.1039/DF9633500137

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Electronic Structure and Spectrum of the HCO, Radical BY T. E. PEACOCK, RIAS-UR-RAHMAN, D. H. SLEEMAN AND E. S. G. TUCKLEY Chemistry Dept., King’s College, Strand, London, W.C.2 Received 8th January, 1963 Additional evidence as to the nature of the emitter of the band system obtained from formic acid vapour points to the radical HCO2 as the emitter. The interpretation of its vibrational structure poses a number of problems. In the hope of finding an explanation of the unusual features of the spectrum, self-consistent field molecular orbital calculations of the energies of the excited states of this radical for selected molecular parameters are now being carried out. Formic acid emits an extensive band spectrum when excited by light in the Schumann ultra-violet or in an electrodeless discharge.1 The bands are always accompanied by the (1E+2II)OH bands and, as obtained in the discharge, by the 4315 CH band and also the CO system if the pressure and rate of flow through the discharge are not large.Whereas the intensity of the CO bands in the discharge is negligibIe for small contact times and increases steadily as the contact time increases, the reverse is true of the formic acid bands which decrease from a maximum intensity at the shortest contact time. The OH band intensity follows that of the formic acid system at small contact times but passes through a minimum and behaves like CO at larger contact times. The behaviour of these three band intensities confirms that the CO bands arise from secondary excitation of CO while the formic acid bands are emitted directly by excited formic acid or an immediate excited dissociation product thereof.The OH bands are emitted at small contact times by excited hydroxyl radicals dissociated from excited formic acid and at larger contact times by secondary excitation of a product, presumably water. The possible emitters of the formic acid bands are (a) HCOOH, (b) HCO, (c) HC02, (d) COOH, (e) C02. Diatomic emitters are excluded as more than two vibration intervals are present. (This is not entirely conclusive, as more than two electronic levels might be involved. However, all the diatomic moIecules that could be derived from formic acid are well known and the formic acid spectrum cannot be associated with any known levels of these molecules and dimeric formic acid is excluded by the observation that temperature variation has no effect on the intensity of the bands.) Substitution of the hydroxyl hydrogen by deuterium does not affect the spectrum (apart from the OH band) but complete deuteration causes small shifts in some of the longer wavelength bands.2 Emission by CO;! or COOH is incompatible with this observation.Similar but much broader bands are emitted by methyl and ethyl formates.2 Re-examination of the spectra and comparison with the band systems subsequently obtained from CH3O and C2Hs0 3 has revealed that the emissions from these esters consist of superimposed formic acid and alkoxyl bands. With methyl formate, 144T . E. PEACOCK, RIAS-UR-RAHMAN, D . H . SLEEMAN, E . S . G. TUCKLEY 145 individual formic acid bands can be distinguished at the visible end of the spectrum and individual CH30 bands are faintly visible at the ultra-violet end.This observation eliminates COOH. The formic acid system consists of longish progressions with an interval of about 1120 9, which must be associated with the ground state or a low-lying state of the emitter. A vibration of 1120 5 associated with a low state of HCO could only be a bending mode and would accordingly show a large isotope effect. By elimination it is therefore concluded that HC02, isoelectronic with N02, is the emitter. The band system consists of two pairs of strong progressions with the interval of 1120 V mentioned above. The high-frequency edges of the first detectable pair lie at 30440 and 30310 V. These pairs are of similar intensity. The first bands of the other progression are at 29838 and 296205; the higher frequency members of the pairs are stronger than the others.There is a background of much weaker bands which also occur in pairs. Weak bands, if any, at frequencies above 305005 would be masked by the hydroxyl band lines. Below 26500V the positions and intensities of the bands become less regular and at least one new progression appears. When examined at a resolving power of 1 5 in the region of 27000 V the bands are entirely diffuse and structureless and it is difficult to measure even the best defined edges to better than 5 V. There is little hope of obtaining further information from the rotational structure of the bands, so that attention has been transferred to the calculation of the electronic levels to be expected for HC02.MOLECULAR GEOMETRY A The molecule is assumed planar with dimensions YCO = 1-25 A and OCO = 120". The molecule has a n electron system made up There is also a PO system which arises from the overlapping of the in-plane 2p orbitals (directed inwards) centred on the oxygen atoms. There are seven electrons in the valence shell, and five avail- able m.o.s, three of n symmetry and two of Q symmetry. The integrals involving the 2pn orbitals are taken from previous w0rk~4-6 and those in- volving the 2pa orbitals have been evaluated semi- empirically except for the two-centre integrals o which have been calculated analytically. Integrals whose values have not been given previously 3-5 are (1) for the n-electron system ~ C O = 0,62630 p, y 0 0 ) = -0.97262 p, and (2) for the Q system pool = -0.001 11 p and GCOO = 2.25000 p, where p = -4.79 eV.4 The molecule, together with the numbering of the atoms, is shown in fig.1. of the 2pn a.0.s of the carbon and oxygen atoms. H I /*= \ 2 , FIG. 1.-The HC02 radical showing the numbering of the atoms. CALCULATION The orbitals were made self-consistent using the Roothaan method for open shells 7 as modified for systems containing only one unpaired electron.$ The configurations which arise from the excitation of one electron only were allowed to interact leading to the ground state and twelve excited configurations, two of which lie very close to the ground state.146 STRUCTURE OF HC02 The s.c.f. calculations give the following m.0.s : TABLE 1 71 ORBITALS. 71 = 0.5399641 + 0.6456542+ 0.5399643, 71' = 2-'(41-43), 7 ~ * = 0*45645#1- O*76353$2+ 0.4564543, 41 and43 are the out-of-plane 2p orbitals on atoms 1 and 3 and42 that on atom 2.B ORBITALS (s == 2-*(#4+#5), B* = 2-j(44-45). 44 and 4 5 are the in-plane 2p orbitals on atoms 1 and 3 respectively. The TC charges and bond orders are PO = 1-0832, PC = 0.8336, PCO = 0.6667. The m.0.s in order of increasing energy are given in table 2. The symmetry of the orbitals under the operations of the point group CzV are also given. TABLE 2 n* bl - 1.192Q no a2 -390p 0* b2 -3728 B a1 *370,8 71 bl 1.4378 The 0, B* and no orbitals are near degenerate. The ground state has the odd electron in the non-bonding x orbital, but the two configurations in which the B and B* have the odd electron are near degenerate to it (the 0 and o* orbitals are separated by about SO?). Setting the energy of the a2 configuration to zero the b2 and a1 configurations have energies of 40 and 120 ij above a2 respectively.SYMMETRY a1 Ql @2 Q 3 Q4 a5 SYMMETRY bl @7 @8 @9 (910 SYMMETRY b2 Qll a12 SYMMFJTRY a2 @6 a13 TABLE 3 orbitals involved energy (in units of -040314 - 1 -58308 - 1.05998 o*ooooo - 1.09624 - 3.56294 - 0.99571 - 1.32180 - 4.4244 1 -4.60885 - 0.00093 - 0.67381 - 1.58088T . E. PEACOCK, RIAS-UR-RAHMAN, D. H . SLEEMAN, E. S . G . TUCKLEY 147 The energies of the ten excited configurations, which arise by excitation of one 7~ electron from the ground state or one of the two low-lying configurations are given in table 3.The first three excited states are mixtures of dD3, @7, @8 and (Dl2 and are : The one-electron transitions by which these three excited states may pass into one of the three ground states (transitions polarized with a2 symmetry excluded) are Y1-+@*1, Y 2 - 4 4 , Y2--+@1, Y3-+@1. The transition energies, their polarizations and oscillator strengths, are given in table 4. u-”l = @ j ; Y2 = -0.81677 @7+0.57696 @8 ; y3 = @12. TABLE 4 transition polarization oscillator strength transition energy %+@ll b2 0.132 25,900 V Y2+@4 b2 0.174 25,920 Y2-41 bl 0~000 25,800 Y3+@l b2 0.1 32 25,900 The next state lies 15,000 V above “1 and Y3 and hence can be excluded from consideration. DlSCUS§ION The experimental data require four transitions forming two almost degenerate pairs.One of these pairs has both members with comparable intensity, whilst in the other pair the high energy member is much more intense than the lower member. From table 4, Y1-411 and Y3-+@1 give one degenerate pair (0-4 band at 25,900 V). This degeneracy will be broken when we take the G-CT interactions into account and may well give a separation of about 120V. They also have identical oscillator strengths and are polarized in the same direction, i.e., parallel to the line joining the two oxygen atoms. The other pair arise from Y!2+@4 and Yz-+@1 and are separated by 120 r? (0-4 band at 25,850 ij). The former has the same polar- ization as the first pair and a comparable oscillator strength, whilst the other is polarized perpendicular to the plane of the molecule and has zero oscillator strength. These four transitions appear to account in a satisfactory way for the observed emission spectrum. The calculations and the interpretation of the observed emission spectrum support the deduction that the emitter concerned is HC02. We are now carrying through non- empirical calculations using this model for the emitter, including all seventeen electrons. We wish to thank Prof. D. W. G. Style for arousing our interest in this problem We also wish to thank Mr. M. P. Melrose for E. S. 6. T. and D. H. S. also wish to acknowledge the award of D.S.I.R. Student- and for many helpful discussions. some experimental assistance. ships during the tenure of which this work was carried out. 1 Terenin and Neujmin, Acta physicochim., 1936, 5, 465. 2 Style and Ward, J. Chcm. Sac., 1952, 2125. 3 Style and Ward, Trans. Faraduy Sac., 1953,49, 999. 4 McWeeny, and Peacock Proc. Physic. Sac., 1957,70,41. 5 Peacock, J. Cheni. Sac., 1959, 3241. 6 Peacock, Proc. Physic. SOC., 1951, 78, 460. 7 Roothaan, Rev. Mod. Physics, 1960, 32, 179. 8 Goodman and Hoyland, J. Chem. Physics, 1962,36, 12.

ISSN:0366-9033    

DOI:10.1039/DF9633500144

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Absorption Spectra of the Hydrides, Deuterides and Halides of Group 5 Elements BY C. M. HUMPHRIES, A. D. WALSH AND P. A. WARSOP Chemistry Dept., Queen’s College, Dundee Received 14th January, 1963 Data are reported on the absorption spectra of PH3, PD3, AsH3, AsD3, SbH3, PF3 and PCl3 and compared with corresponding data for NH3, ND3. All the electronic transitions observed are Rydberg transitions leading to the first ionization potentials; and all with discrete structure are represented by long progressions in the upper state bending frequency (v2) alone. The regularity of all the progressions observed is compatible with the upper states of the molecules being (i) planar; or (ii) pyramidal with such a low barrier to inversion that all the transitions observed are to vibra- tional levels well above the top of the barrier; or (iii) pyramidal with such a high barrier to in- version that all the transitions observed are to vibrational levels well below the top of the barrier.However, consideration of the energies required to change the inter-bond angles in the PF3 and PCl3 molecules from their excited-state values to their ground-state values leads to the conclusion that whereas (i) applies to the Rydberg states of NH3 (and the ground state of the NHS ion), (iii) applies to the Rydberg states of PF3 and PCl3 (and the ground states of the PF; and PClf ions). It is stressed that whereas the spacing of the observed bands for NH3, ND3, PF3 and PCl3 is -v;. the spacing of the observed bands for PH3, PD3, AsH3, AsD3 and SbH3 is -v;/2. The contrast between v;/vl for the latter group of molecules and for PF3, PC13 on the one hand and NH3, ND3 on the other is used as an argument that neither (iii) not (i) applies to the upper states of PH3.. . SbH3 and that these molecules probably have Rydberg states (and positive ion ground states) described by (ii). A previous paper 1 dealt with the ultra-violet absorption spectrum of ammonia. Five separate electronic transitions were recognized, each of Rydberg type and each giving rise to a long progression of bands in the excited state vibration (v;) that causes a symmetrical change of the HNH angles. Starting with the longest wavelength transition, we shall refer to these five transitions as the first, second, . . . transitions of ammonia. The origins of the first, third and fourth transitions lie at 2168, 1434 and 1330A.The origin of the second transition was thought to lie at 1665 A, but has since been shown by Douglas and Hollas 2 to lie one quantum of vi to longer wavelengths, i-e., at 1689A. The first four transitions were each proved,l by a vibrational analysis, to have planar upper states. Douglas and Hollas 2 have since confirmed, by a rotational analysis, that the second transition has a planar upper state. The upper state of the Mth transition (of unknown origin, but represented by bands in the neighbourhood of 1268A) and, indeed, the upper states of all the Rydberg transitions of ammonia (including the ground state of the NH; ion) were concluded to be also planar. Certain weak parallel bands observed by Douglas and Hollas 2 between 1546 and 1458 A may represent a further electronic transition or possibly a v; + nv; progression belonging to the second (1 689 A) transi- tion. Other Rydberg transitions must occur as the ionization potential at 1221 A is approached.The stronger bands of the second transition are perpendicular and the upper state electronic symmetry is E”.2 The upper state of the first transition has A; electronic symmetry 1 and the bands observed should be parallel. In the present paper the absorption spectra of PH3, PD3, AsH3, AsD3, SbH3, PF3 and PC13 are discussed. All the spectra were photographed with the aid of a 2-m vacuum spectrograph. The dispersion was ca. 8-7 A/=. 148C. M. HUMPHRIES, A . D. WALSH AND P. A. WARSOP 149 RESULTS The full data for the hydrides and deuterides will be published elsewhere; a preliminary account has already been given.3 The spectra of deutero phosphine and phosphorus trichloride are reproduced on plate I ; the spectrum of phosphorus trifluoride is shown on plate 11.The lowest energy absorption transition (see plate 11) of phosphorus trifluoride consists at low pressures, of two very diffuse peaks, one at 1564 A and one at 1515 A. TABLE TH THE 1405 A PROGRESSION OF PHOSPHORUS TRIFLUORIDE frequency (cm-I) Av (cm-1) ca. 71,174 ca. 71,661 72,144 72,602 \ca. 487 ca. 483 73,958 75,360 76,717 77,174 > 456 77y6307 463 78,093 -< 467 78,560 \ 452 79,012- > 457 79,455 - TABLE THE 1212 A PROGRESSION OF PHOSPHORUS TRIFLUORIDE frequency (cm-1) Av (cm-1) 82,511 7 470 472 82,981 ,-< 451 459 456 474 83,453 / 83,904 -< 84,363 -< 84,819 ___( 85,749 85,2g33 456 \ 465 \ ca.460 ca. 457 4 463 451 462 86,214 2 ca. 86,674 87,131 87,594 -< 88,045 / 88,507 A As the pressure is increased, these rapidly merge into each other and the resulting continuum spreads out slowly and symmetrically to long and short wavelengths. At high pressures, the short wavelength side of the continuum overlaps a long pro- gression of rather diffuse bands which first increase and then decrease in intensity150 SPECTRA OF HYDRIDES, ETC., OF GROUP 5 ELEMENTS as they proceed to short wavelengths. The first observed band is at 1405A and maximum intensity occurs around 1300A, i.e., at about the thirteenth band. As may be seen from plate 11, the envelope of this second, banded absorption region resembles, but has a lower peak intensity than, the envelope of the first, continuous absorption region.The short wavelength side of the second absorption region TABLE 3.-THE 1591 A PROGRESSION OF PHOSPHORUS TRICHLORIDE frequency (cm-1) 62,823 63,089 63,361 63,619 63,879 64,152 4) 64,680 64,411 2’ \ :: > \ 64,931 -------< 65,197 65,464 65,734 < 65,985 ~ 66,231 66,478 - > i 66,778 67,025 Ar(cm-1) 266 272 258 260 273 269 269 25 1 266 267 270 25 1 246 247 300 247 overlaps a further progression of diffuse bands beginning at 1212A. These further diffuse bands increase in intensity as they proceed to short wavelengths; they can be followed up to about the fourteenth band (at ca. 1130A), but thenceforward merge into strong continuous absorption.The third absorption region is somewhat stronger than the second and similar in intensity to the first absorption region. Frequency measurements of the centres of the bands in the 1405 and 1212 A progressions are given in tables 1 and 2 respectively. The diffuse- ness of the bands and the occasional presence of emission lines in the Lyman continuum used as background make precise measure- ment impossible and imply that no stress should be put upon the deviations from constancy of the band separations in the two progressions. The mean band separation in the 1405 A progression is 460 cm-1 and in the 1212 A progression is 461 cm-1. The transitions are so strong that they must be allowed. Assuming the transitions do not reduce the three-fold symmetry of the ground state, then the frequencies involved must be totally symmetrical with respect to C3v sym- metry, i.e., v; (symmetrical P-F stretching) or vi (symmetrical bending).v; and v; are, respectively, 892 and 487 cm-1.4 The mag- nitudes of the frequencies involved then leave no doubt that the frequencies represent v;. One notes that v2 is but little reduced by the electronic excitation. The lowest energy absorption region of phosphorus trichloride consists of a continuum of maximum intensity around 1750 A. With increasing pressure, this continuum spreads to both red and violet and on the violet side overlaps the second electronic transition. The latter consists of a long progression of diffuse bands which mount in intensity and, after the seventeenth band, merge into continuous absorption whose intensity is a maximum around 1470A and fa, 11s off towards shorter wavelengths.The first observed band is at 1591 A. The measured fre- quencies and frequency differences of the bands are given in table 3. The mean band separation is 263 cm-1. The frequency involved is undoubtedly vi; IJ; is 240 cm-1.5 Again one notes that v2 is changed very little by the electronic excitation. The difficulties of measurement again mean that the deviations from constancy ofPLATE 1 .-The absorption spectra of (a) trideutero phosphine, (b) phosphorus trichloride. PLATE 2.-The absorption spectrum of phosphorus trifluoride. [To face page 150.C. M. HUMPHRIES, A. D. WALSH AND P . A. WARSOP 151 the band separations shown in table 3 are not necessarily significant.To short wavelengths of the 1591 A progression and associated continuum, is a further con- tinuum, the intensity of which rises rapidly to a maximum and then decreases gradually. This continuum has the appearance of a very broad, violet-degraded, band and has maximum intensity at 1385 A. At the pressures used (up to ca. 1 mm Hg in a 4 m path), the lowest energy ab- sorption transition of phosphine or deutero phosphine (see plate I) consists of a continuum of maximum intensity at 1800 A.* On its short wavelength side, this continuum overlaps a long progression of bands which has a higher appearance pressure than does the peak of the continuum. The PD3 bands are sharper than those of PH3. Nine PH3 bands have been measured and fit the formula Twelve PD3 bands have been measured and fit the formula Because of the overlap by the 1800 A continuum, the first observed band with either PH3 or PD3 is not necessarily the origin of the progression.On the short wave- length side, each progression is overlapped by further absorption. The observed bands of progressions (1) and (2) do not change markedly in intensity with wave- length and the overlap with other (stronger) electronic transitions to both long and short wavelengths makes it impossible to determine the position of maximum intensity. The further absorption that overlaps the short wavelength side of the second electronic transition consists of several overlapping progressions. With PH3, the bands are very diffuse and it is impossible to disentangle the various pro- gressions with certainty.The first observed band lies close to 15OOA and band separations increase from about 420cm-1 at the long wavelength end to between 500 and 600 cm-1 at shorter wavelengths. The first PD3 band which does not belong to progression (2) was earlier 3 observed at 69,716 em-1, but bands have now been traced back to 67,532 cm-1. The band separations are, initially at least, ca. 400 cm-1. A further progression of 14 PD3 bands fits the formula Each of the progressions observed for PH3 and PD3, like all those observed for NH3, is undoubtedly a progression in vh. v; has the values 992 and 730 cm-1 respectively, for PH3 and PD3.9 It thus appears that all the electronic transitions observed for PH3 and PD3 roughly halve the value of v2.The spectra of AsH3 and AsD3 are very similar to those of PH3 and PD3, but are more diffuse. The first, continuous absorption region has a maximum at about 1830 A. The second electronic transition, with AsH3, is represented by a progression of twelve bands obeying the formula and, with AsD3, by another progression of twelve bands obeying the formula v (cm-1) = 62,801 +488.0 n’+7.84 n’2 (n’ = 0, 1, 2, . . ., 8). v (cm-1) = 62,865 + 361.6 n‘ + 4.0 n’2 (n’ = 0, 1, 2, . . ., 11). (1) (2) v (cm-1) = 74,946+356-4n’+2.11 12’2 (n’ = 0, 1, 2,. . ,, 13). (3) v (em-1) = 62,453-1-421.5 n‘+5.94 n’2 (n’ = 0, 1,2, . . ., ll), v (cm-1) = 63,057 + 319.0 n’ +2.70 n’2 (n’ = 0, 1,2, . . ., 11). (4) (5) * Using 760 mm Hg of phosphine (prepared from pliosphonium iodide and alkali) in a path length of 110 cm, Cheesman and Emelbus 6 observed two very faint, diffuse bands, at 231 5 and 2290 A.The bands were independently observed by Melville.7 Since the separation of the bands is 470 cm-1 (i.e., of the same magnitude as the band separations in the shorter wavelength transitions), it seemed plausible that the bands were due to phosphine and not to an impurity. Dr. L. Mayor, however, working in this Department and using phosphine prepared from magnesium aluminium phosphide, has been unable to reproduce the bands.152 SPECTRA OF HYDRIDES, ETC., OF GROUP 5 ELEMENTS Both progressions are overlapped at their long wavelength end by the stronger 1830 8, continuum ; and so the first observed bands may not be the progression origins.A further progression of AsD3 can be recognized; it obeys the formula v (cm-1) = 77,098+313-6 n'+2.43 n'2 (n' = 0, 1,2, . . ., 9). (6) Between progressions (5) and (6) of AsD3, fragments of at least two other progres- sions, with band separations - 350-400 cm-1, can be recognized ; and there are signs of AsH3 progressions to short wavelengths of progression (4). The band separations in all these progressions of AsH3 and AsD3 undoubtedly represent vi. vi has the values 906 and 660cm-1 respectively for AsH3 and AsD3.10 All the banded elec- tronic transitions observed thus result in (roughly) halving v2. The first transition of stibine is represented by a continuum of maximum intensity at about 1970A; and the second by a progression obeying the formula v (cm-1) = 58,320+419-4 n'+4-94 n'2 (n' = 0, 1,2, .. ., 7). (7) Very diffuse bands to short wavelengths of this progression represent a third elec- tronic transition. Still further to short wavelengths occurs a transition represented by a progression obeying the formula, v (cm-1) = 77,395+419 n'-4*4 n'2 (n' = 0, 1, 2. . . ., 7). vi for SbH3 is 796 cm-1.11 It follows that, with SbH3 as with PH3, PD3, AsH3 and AsD3, the banded electronic transitions observed result in a rough halving of v2. DISCUSSION The halide, hydride and deuteride spectra described all have considerable similarities. In each spectrum, the second electronic transition is weaker than the first and also weaker than some of the shorter wavelength transitions. The first electronic transition in each spectrum is predissociated; with NH3 weakly, so that vibrational structure is retained and only rotational structure is lost ; with the other molecules so strongly that all structure (except in the spectrum of PF3, the two diffuse humps at 1564 and 1515 A, whose interpretation is unknown) is lost.Each discrete transition is represented by a long progression in v; and there is no evidence of any excitation of v;. Excitation of long vi progressions implies that the electron concerned comes from the lone pair orbital (a1 with respect to C3v symmetry) on the group 5 atom, which orbital is the most weakly bound orbital occupied in the ground state and is approximately non-bonding. It is the presence of two electrons in the a1 lone pair orbital that causes the ground states to be pyramidal.The absence of excitation of v; implies that each transition causes little change in the bonding between the central atom and the other atoms ; and, therefore, that the electron concerned passes to a Rydberg orbital, which is the only type of orbital, unoccupied in the ground state, that is expected (because of its size) to be without considerable influence on the molecular dimensions. All this is made clear by the diagram plotted by one of us 12 correlating the orbitals of a pyramidal AH3 (or AB3) molecule with those of the same molecule in a planar configuration. The upper states may thus all be described as Rydberg states leading to the first ionization potential; and the properties of the various upper states for any one molecule should differ little from state to state and from the ground state of the ion AH: or ABZ.Clearly, too, the inter-bond angles in all the upper states and in the ions must be considerably greater than in the ground states. With noC. M. HUMPHRIES, A . D . WALSH AND P. A . WARSOP 153 transition of any of the molecules we are dealing with have we observed alternate long and short spacings between the bands. This is consistent with one of three possibilities. The first is that all the observed bands for any particular molecule represent transitions to a planar upper state. The second is that all the observed bands represent transitions to vibrational energy levels lying well below the top of the barrier to inversion of a pyramidal state. The third is that all the observed bands represent transitions to vibrational energy levels lying well above the top of the barrier to inversion of a pyramidal state.With NH3 we have already con- cluded that the upper states and the ion NH< are planar. It does not, however, necessarily follow from the similarities between the spectrum of ammonia and the spectra of all the other molecules studied that with every molecule the inter- bond angle in the upper states is increased by the electronic excitation as far as 120", i.e., that the first possibility holds. In any particular excited state of a molecule AB3, the energy required to bend the BAB angles from their equilibrium value in the excited state to their value in the ground state is given by the separation of the origin and the position of maximum intensity in the relevant transition.Thus, with the first Rydberg transition of NH3, maximum intensity occurs 1 at about v' = 6 (51,550 cm-1) and the origin certainly at 2168 A (46,181 cm-1) ; so that the energy required to bend the excited state from planarity to a pyramidal form in which the HNH angle has the ground state value of ca. 107" is about 5400cm-1. With the second Rydberg transition of NH3, maximum intensity 1 occurs at about v' = 5 (64,981 cm-1) and the origin 2 certainly at 1689 A ; so that the separation is about 5800 cm-1. The rough equality of the two calculated energies agrees with the expectation that all the Rydberg states should have properties that are very similar and approximate to those of the ion. One would expect that the bending energy would be rather more in the second than in the first excited state, since the vibrational frequency (and hence the bending force constant) are somewhat greater in the second than in the first excited state; the above rough estimates agree with this.For the 1434 and 1330 A transitions, ac- cepting the origins and positions of maximum intensity given previously,l the calculated energies are rather less ( N 3800 cm-1). The vibrational frequencies, and hence the bending force constants, would lead one to expect bending energies greater than that for the first excited state; and there may be an indication here that the true origins of the third and fourth transitions lie one or two quanta of v;l to the red of the positions hitherto given, or more probably that the positions of maximum intensity (which are difficult to determine with precision) lie one or two quanta of v; to the violet of the positions previously given.It is noteworthy that if one adds two quanta of v; to the previously given separations of the origins and positions of maximum intensity, one obtains bending energies that are - 5600 and - 5700 cm-1 for the upper states of the 1434 and 1330 A transitions respectively. The bending energies are then all very similar and their small variation qualitatively accords with the variation in vi frequency and deduced variation in bending force constant. Thus the difference between the adiabatic and vertical first ionization potentials may be -0.7 eV rather than the previously given value of -0-5 eV. With PF3, the separation of the first observed band and the position of maximum intensity of the second electronic transition is about 5500 cm-1; while the separations of the first and last observed bands in the third electronic transition is 5996 cm-1. These data and the apparently similar envelopes of the first and second transitions are consistent with the expectation that the FPF angle in each upper Rydberg state is, at least roughly, the same; and with the first observed bands of the second and third transitions lying within a few quanta of 460 cm-1 from the origins.With154 SPECTRA OF HYDRIDES, ETC., OF GROUP 5 ELEMENTS these assumptions, the energy required to bend, without change in P-F length since v; is not excited, the FPF angle from its equilibrium value in the Rydberg states of PF3 (and in the ion PF;) to its value in the ground state is within a few multiples of 460cm-1 from 5500cm-1.In other words, the difference between the vertical and adiabatic first ionization energies of PF3 should be ~ 0 - 7 eV. Let us assume that the upper states of the observed transitions of PF3 are planar. The valence force expression for the potential energy V of a PF3 molecule that is planar in its equilibrium form, as a function of (i) changes (Q) in the P-F distances, (ii) changes (6)in the FPF angles (iii) changes (A) in the angle between a PF line and the F3 plane, is (ref. (13), p. 178) : 2V = ki<Q?2 + Q?3 + Q?4> + k d d k + 6224 + 6 L ) + k~(A?2 +A?3 +A:4). kl, Icb and kA represent the appropriate force constants. Al~0,13 4n2v;c2 = I+-- -2, ( ::)n: where l is the P-F length and the other symbols have obvious significance. v; is not excited in the observed transitions, we may take l as approximately the same as the ground state P-F length.Electron diffraction and microwave data 1.1 are consistent with the ground state FPF angle being 100" and P-F length being 1-53(5) A. The energy required to bend symmetrically, without chaFge in P-F length, the supposedly planar state to a pyramidal form in which FPF = loo", should, from eqn. (9) and (10) be given by where 6 = 20/57-3 radians and, since FfiF = 100" corresponds to an angle of 28" between an F-P line and the F3 plane, A = 28/57-3 radians. To evaluate kb for the planar molecule requires a knowledge of v; and vi, which are unknown.How- ever, the first term on the right-hand side of eqn. (11) is expected to be positive and the second term alone leads, on inserting numerical quantities including vi = 460 cm-1, to an energy of ca. 36,000 cm-1. Since the calculated energy is so much greater than ca. 5500cm-1, then it is improbable that the Rydberg transitions of PF3 lead to planar upper states. Since the separation of the first and last bands observed in the second electronic transition of PF3 is only 79,455-71,174 = 8281 cm-1, then all the vibronic transi- tions observed take place to upper state vibrational levels that probably lie well below the top of the potential barrier to inversion; and the absence of any measurable inversion effects is understandable. Assuming then pyramidal upper states, we may estimate the FPF angle in these states as follows.The energy required to bend the three FPF angles in a pyramidal PF3 molecule by an amount 6 without change in the P-F distances should be given approximately by the expression v = $kad2, (12) (ref. (13), p. 175). k6/P in the ground state is known, from the v; and v; frequencies, to be 1-15 x 105 dynes/cm (ref. (13), p. 177). As above, we shall take l in the excited state to be the same as in the ground state. The observed values of vi are prac- tically the same (460 cm-1) and only slightly less than the value of v; (487 cm-1). The force constants controlling bending may, therefore, be taken as approximatelyC. M. HUMPHRIES, A . D . WALSH AND P. A . WARSOP 155 the same in the ground and excited states and more precisely as in the ratio kL/k: = (460/487)2.Inserting the value V = 5500 cm-1 in eqn. (12),* we then find that the equilibrium FPF angle in the Rydberg states should be, roughly, only 110". Very similar arguments and conclusions apply to PC13. Here microwave data yield an angle of 100" and a bond length ( I ) of 2.04(3) A in the ground state ; 15 while force constant calculations, based on v; and v;, yield ka/P = 0.43 x 105 dynes/cm for the pyramidal ground state. as the position of maximum intensity and assuming the 1591 A band to lie within a few vi quanta of the origin, one obtains ~ 5 2 0 0 cm-1 for the difference between the vertical and adiabatic first ionization energies and an upper-state equilibrium angle of roughly 11 lo.* Similar calculations and arguments cannot be applied to PH3, AsH3 and SbH3 because for no transition of these molecules is the separation of the origin and the position of maximum intensity known with sufficient certainty. However, a major reason for Rydberg excitation of PF3 and PC13, unlike NH3, failing to produce planar molecules is that the ground state inter-bond angles are considerably smaller than in NH3.It would, therefore, not be surprising if Rydberg excitation of PH3, AsH3 and SbH3 (which molecules have ground state inter-bond angles of only 93" or less) failed to produce planar upper states. In this connection, however, although the lowest energy, excited state of the NH2 free radical is linear, that of the PH2 radical is probably non-linear.16 On the other hand, it would seem likely that Rydberg excitation of PH3, AsH3 and SbH3 will produce molecules with inter-bond angles at least approaching the value of the HNH angle in the ground state of NH3; and therefore that the upper states should show inversion effects.These have not been observed, but may be present in thus far unobserved transitions. If so, it is only likely that they occur in transitions close to the (unobserved) origins ; if they occurred in transitions of higher energy than the shortest wavelength members of the various progressions, the energy range covered by the observed progression bands would imply barriers to inversion much higher than the known barrier height in the ground state of NH3. In this connection also, there is one outstanding difference between the transitions of NH3, ND3, PF3 and PCl3 on the one hand and the transitions of PH3, PD3, AsH3, AsD3 and SbH3 on the other (see ref. (3) for a discussion, confined to the hydrides, of the difference).With each of the former molecules, the values of v; in the various transitions are all of about the same magnitude as the value of vi. [For NH3, vl here refers to the separation of the (slightly split) levels well below the top of the barrier to inversion.] With each of the molecules PH3, PD3, AsH3, AsD3 and SbH3, however, the observed values of vi are close to one-half the value of vi. The contrast between the latter molecules and PF3, PC13 is explicable if we assume that the upper states of PH3, etc., differ from the upper states of PF3, PCl3 in having a barrier to inversion that is of low or zero height ; and if we assume that the observed transitions of PH3, etc., are all to levels well above the top of any inversion barrier, whereas the observed transitions of PF3, PCl3 (as we have concluded) are all to levels well below the top of the barrier.For any particular electronic state, the spacing of the vibrational levels well above the top of the barrier to inversion should be approximately one-half of the spacing of the levels well below the top of the barrier. If, therefore, electronic excitation changes but little the vibration frequency vz except For the second electronic transition, taking 1470 * The interval between the estimated position of maximum intensity and assumed origin only enters these calculations as the square root; even if the interval used is in error by 50 %, the cal- culated angle is only changed by 2-t".156 SPECTRA OF HYDRIDES, ETC., OF GROUP 5 ELEMENTS in so far as inversion effects are concerned (as seems true for PF3 and PC13), the fact that the observed bands of PH3, etc., have a spacingv;/2, whereas the observed bands of PF3, Pc13 have a spacing -v; is understandable. The difference between the two groups of molecules is that for PF3 and PC13 we have observed only transi- tions to levels below the top of the barrier to inversion whereas for PH3, etc., we have observed only transitions to levels above the top of any barrier.Conversely, one may argue that the halving of frequency for PH3, etc., constitutes evidence that the excited states of PH3, etc., do not possess a high barrier to inversion and, unlike PF3 and Pc13, are not simply pyramidal.If the excited states of PH3, etc., were simply pyramidal, one would have to make the unplausible assumption that corresponding electronic excitation makes little difference to the force constant controlling bending in PF3, Pcl3 but reduces the force constant for PH3, etc., to one-quarter of its ground state value. We conclude, then, from the contrast of PH3, etc., with PF3, PC13, that the ex- cited states of PH3, etc., are either strictly planar or pyramidal with a low barrier to inversion. In themselves, the observed spectra of PH3, etc., are consistent with either possibility. However, the contrast between PH3, etc. (where v; - vi/2) and NH3 (where v;-vi and the upper states are certainly planar) then constitutes an argument that the upper states of PH3, etc., are not completely planar and, by elimination, must be pyramidal with a low barrier to inversion.If they were com- pletely planar, the striking difference between the upper state spacings of PH3, etc., and those of NH3 would be correlated with no other outstanding difference and it would seem particularly difficult to understand the contrast in values of v&. To this argument we may add the points made above concerning the very large increases of angle needed to make PH3, etc., planar and concerning the contrast of PH2 and NHz. Admittedly, it is not easy to see why, for NH3, vi should be ~ v ; . Since the electronic excitation apart from inversion effects does not greatly change the v2 vibration frequency in PF3, in Pc13 and (as is necessary for our explanation of why v;-v;/2) in PH3, etc., one might expect that with NH3 v; would be 4 2 .It seems that once there is no double minimum in the potential function and the excited states are strictly planar, some new effect comes in that roughly doubles the vi frequency in the excited states of NH3 from its expected value. As we have argued before,3 the dificulty lies with NH3. One possible explanation is as follows. The lone-pair electron which is excited in all the electronic transitions dealt with here has a tendency to bend the molecule from a planar to a pyramidal shape.12 The removal of a lone-pair electron from a pyramidal molecule such as PF3 causes the bond angle to increase without apparently changing the vibrational frequency v2 very much.The removal of a lone-pair electron from a planar molecule (e.g., from NH> to form NH:+), however, cannot change the bond angles (see ref. (12)), but might be expected to make it more difficult to bend the molecule, i.e., the vibrational frequency might be expected to increase when no change of shape is possible. NH3 in its ground state is so near to being planar that the removal of a single lone-pair electron may be sufficient not only to make the molecule planar (with a frequency 4-1~72) but also to cause the frequency to increase until it is almost equal to vi. In other words, all the observed spectra can be understood if (a) the removal of a lone-pair electron from a pyramidal molecule causes the bond angle to increase but, provided the molecule does not become planar, makes little change in the vibrational frequency v2; (b) the removal of a lone-pair electron from a planar molecule causes the frequency v2 to increase; (c) the Rydberg states of PF3, PCl3C. M. HUMPHRIES, A . D . WALSH AND P . A . WARSOP 157 are pyramidal with a high barrier to inversion; (d) the Rydberg states of PH3, PD3, AsH3, AsD3 and SbH3 are pyramidal with a low barrier to inversion ; (e) NH3, ND3 are planar in their Rydberg states. 1 Walsh and Warsop, Trans. Faraday SOC., 1961, 57, 345. 2 Douglas and Hollas, Can. J. Physics, 1961, 39, 479. 3 Walsh and Warsop, 4th Int. ConJ Molecular Spectroscopy, Bologna, 1959 (Pergamon Press, 4 Wilson and Polo, J . Chern. Physics, 1952, 20, 1716. 5 Davis and Oetjen, J. Molec. Spectr., 1958, 2, 253. 6 Cheesman and Emelkus, J. Chem. SOC., 1932, 2847. 7 Melville, Nature, 1932, 129, 546. 8 Thompson and Duncan, J. Chem. Physics, 1946, 14, 573. 9 McConaghie and Nielsen, J. Chern. Physics, 1953, 21, 1836. 10 McConaghie and Nielsen, Physic. Rev., 1949, 75, 633. 11 Haynie and Nielsen, J, Chenz. Physics, 1953, 21, 1839. 1 2 Walsh, J. Chem. SOC., 1953, 2296, 2301. 1 3 Herzberg, Infra-red and Ramaiz Spectra (Van Nostrand, New York, 1945). 14 Williams, Sheridan and Gordy, J. Chem. Physics, 1952, 20, 164. 15 Kisliuk and Townes, J. Chem. Physics, 1950, IS, 1109. 16 Ramsay, Determination of Organic Structures by Physical Methods, vol. 2 (Academic Press, Oxford, 1962). New York, 1962).

ISSN:0366-9033    

DOI:10.1039/DF9633500148

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

Electronically Excited States of Ammonia BY A. E. DOUGLAS Division of Pure Physics, National Research Council, Ottawa, Canada Received 1 1 th January, 1963 The ultra-violet absorption bands of 14NH3, 14ND3 and 15NH3, which lie between 1400 and 2200 A, have been photographed at high dispersion and the rotation structures of a number of the bands have been analyzed. The Zeeman effect has also been studied for a number of the bands. It is shown that in the lowest excited state, which is of A; symmetry, that the molecule is planar with a bond distance of 1.08 A. The Zeeman studies show the second excited state to be degenerate. A third excited state which is the upper state of parailel bands near 15008, is not fully understood. The electronic states and predissociations of NH3 are discussed.The ultra-violet absorption spectrum of ammonia consists of a large number of band systems extending from 2200A into the far ultra-violet. Earlier work has been reviewed and a number of new aspects of these band systems have been dis- cussed in two recent papers by Walsh and Warsop 1 and by Douglas and Hollas.2 In the present work, absorption bands involving three excited states will be discussed. The first of these excited states, denoted * by A, is the upper state of the 2000 A absorption bands.1 The second excited state is the upper state of the 15OOA bands which have been analyzed by Douglas and Hollas, while the third state gives weak bands in the same spectral region as the B-x system. The rotational structures of a number of the c-2 and 2-x bands have been analyzed and the Zeeman effect of the lines of the B-x bands has been observed.Finally, theoretical interpretations for some of the properties of the excited state are discussed. EXPERIMENTAL All spectra were photographed with a 10-m, concave grating spectrograph3 at dis- persions between 0.18 and 0.28 A/mrn. The continuous background was obtained from a Lyman source and lines from an iron hollow cathode lamp were used as wavelength standards. The absorption tube was designed in such a way that it could be cooled to -80°C or heated a few hundred degrees above room temperature. The Zeeman spectra were obtained at a field of 58,000 oersted in an apparatus described previously.4 The ND3 was obtained from Merck and Co., and the 15NH3 was prepared from K15No3 (supplied by Isomet Corp.) by the action of Devardas alloy in NaOH solution.RESULTS 2-2 BANDS The A-x system of ammonia consists of a long progression of bands near 2000 A. The bands of NH3 are too diffuse to show measurable rotational structure. *It is convenient to designate each electronic state by an empirical sy_mbol. Following the notation adapted for diatomic molecules, the gtound state is denoted by X and excited states of the ssme multiplicity as the ground state by ABC . . . . States of different multiplicity are denoted by Zbi.. . . . The tilde is used to differentiate the empirical symbols from those denoting symmetry species. 158A . E. DOUGLAS 159 From a study of the vibrational structure of the bands, Walsh and Warsop 1 have shown that in the upper state the molecule is planar and that the progression of bands arises from excitation of the out-of-plane vibration v;.In an early paper Benedicts pointed out that some of the corresponding bands of ND3 are discrete and, without giving details, stated that a partial rotational analysis shows that, in the upper state, the molecule has the configuration of a planar equilateral triangle with a bond distance r(N--H) of 1.07A. The present work was undertaken in order to obtain a more complete analysis of the discrete bands of ND3. The spectra show that bands involving 0; = 0 have rotational structures con- sisting of broad lines while those involving v; = 1 have much sharper lines. No measurable rotational structure can be seen in bands with 4 2 2 .Fig. la and lb show the 0-1 and 1-0 bands. (The vibrational quantum numbers given here and throughout this paper are the values of v2, all other vibrational quantum numbers being zero.) The 1-4, 1-1, 0-0, 0-1, 0-2 and 0-3 bands have been measured and the wave-numbers of the lines are given in table 1. The analysis of the bands presented no difficulty. In a parallel band of ND3, the first lines of the Q branches of the sub-bands are the strongest lines in the band and this series of lines Q(J = K ) can be identified readily. Also the first few lines of the R branches are free from overlapping and are easily identified. From these lines, approximate rotational constants can be calculated and thereafter the remaining lines can be identified. The energy levels of the u; = 1 state were established by adding the wave-numbers of the observed lines in the 1-1 and 1-4 bands to the appropriate lower state energy levels which were calculated from the constants given by Benedict and TABLE l.-WAVENUMBERS OF THE LINES OF THE A - 2 BANDS OF ND3 1" BAND v (cm-1) branch(J)K v (cm-1) branch(J) K 47334.18 * 308.57 * 280.42 * 377.13 385-72 * 401.11 * 345-76 334.18 * 366.35 385.72 * 393.35 * 401.11 * 364- 12 393.35 * 3 19.61 306.28 292.60 278.02 263.16 360-45 357-35 398.85 47304.09 * 290-44 275.95 260.63 355.56 351.81 287.75 273.16 257.92 349.35 34509 269.76 * 254.47 * 341.85 336-63 327.09 316-19 31 1.36 304-09 * 298.70 28476 * 269.76 * * overlappedI60 STATES OFNH~ Y (cm-1) 46608.79 * 584.78 * 560.38 * 532.79 * 636.68 652.71 * 596.98 * 584.78 * 572.93 * 560-38 * 546.12 * 532.78 * 617.18 636.68 * 645-32 652.71 584.78 572.93 560.38 546- 12 532.78 614.98 645.32 652.71 570.89 558.10 * * overlappedA.E. DOUGLAS 161 Plyler.6 The upper state levels were fitted to the usual equation for the rotation energy levels of a symmetric top and the rotational constants B and C determined. The accuracy of the data, which is limited by the line width, was insufficient to deduce the three centrifugal stretching constants DJ, DK and DJK. The constants which have been determined are given in table 2. F(J, K ) = BJ(J+ 1) + (C- B)K2, (1) TABLE CONSTANTS DERIVED FROM THE A - 2 BANDS OF ND3 0'- "I' 1" 1-1 1-2 0 4 0-1 0-2 0-3 2 4 v ; = l ? vo (cm-1) 473 67.35 46617.88) 45937.0 4671 3.8 45967.4 45355-1 44887.1 48037 48908 vibronic transition B' (cm-1) C' (cm-1 A;-A," 4.78 2-32 A;-A; A',--A! AZ-A; Ai-AI Ai-A; A;-A; diffuse diffuse The large line widths in bands which have u; = 0 as their upper state, prevent useful rotational constants from being determined for this level.Most of the bands arising from vibrationally excited levels of the ground state have u$ = 0 as their upper state and as a result they yield no useful rotational constants and even their band origins may be in error by a few tenths of a wave-number unit. No attempt was made to measure the diffuse bands with ~ $ 2 2 but there is a weak band of ND3 at 48908 cm-1 which is noL a member of the strong progression of bands. First, spectra photographed at room temperature show the 0-0 and 1-1 bands of ND3 have about the same intensity in spite of the fact that the population of the ug = 1 level is only 0.028 of the population of the t$ = 0 level. This Franck-Condon effect has been noted in the spectrum of NH3 by Walsh and Warsop 1 but the effect is even greater in ND3.Secondly, the relative intensities of the first lines of the R branches are very different in bands with = 0 from bands with u; = 1. In the 0-0 band, for example, R(0) and R(l) are roughly of the same intensity whereas in the 1-1 band the A(1) line is strong and the R(0) line is extremely weak. It will be shown later that this difference in line intensities is very useful in determining the symmetry of the vibronic levels. Two unusual characteristics of the A-r?( system are worth noting. B-2 SYSTEM The &--x system consists of a long progression of perpendicular bands between 1690 and 1400A.The analysis of these bands in NHJ which has been given by Douglas and Hollas,2 shows that the vibronic levels of the upper state are degenerate and there is strong evidence that the degeneracy is electronic.* Furthermore, the * A correction of an error in eqn. (3) of their paper has led to a better fit of the data with only minor changes in the constants. Their eqn. (3) should read 7 F162 STATES OF NH3 molecule in the excited state has a large internal angular momentum, and, since this is assumed to arise from motion of the electrons, it should have a large magnetic moment. The rotational levels of the state should therefore show a strong Zeeman effect and experiments were undertaken to test this point.The theory of the Zeeman effect for a linear molecule has been given by Crawford.8 By a simple extension of his equations, it follows that a magnetic field H will displace a rotational energy level of a singlet E electronic state of a symmetric top by an amount gKMH = 4 6 X l o - - J(J + 1) cnl- '. gefzKMH 2mc J( J + 1) AV = In this equation J, K and M are the rotational quantum numbers, e, k, rn and c have their usual meanings and g is the usual Land6 factor which has a value of unity for a molecule with a magnetic moment of one Bohr magneton. It follows from the form of eqn. (2) that rotational levels with K not very different from J are sensitive to the magnetic field whereas levels with J$ K are relatively insensitive. Experimentally the Zeeman effect at a field of 58,000 gauss was examined in the 1 4 , 2 4 and 3-0 bands of the B---X system and a number of bands of the c-z system.A part of the 3 4 band photographed first with and then without the magnetic field is shown in fig. 2a and 2b. All lines in the bands of the &2? system are broadened by predissociation and therefore it was not possible to resolve the Zeeman pattern of any of the lines. As a result, the experiment is qualitative rather than quantitative. There is no doubt, however, that some of the lines are greatly broadened by the magnetic field whereas others are almost unaffected. Furthermore, qualitatively at least, the broadening of the lines depends on the ratio of K to J in the way one would expect from eqn.(2). A rough value of g has been obtained by measuring the widths of a number of lines in each of three bands. The average of these measurements is g = 0.6. The lines of the c-x system showed no measurable Zeemar effect. e-x SYSTEM Douglas and Hollas 2 have described a progression of weak bands of NH3 which lie in the same region as the stronger B--Z bands and they show on: of the bands in plate 1 of their paper. Some of these bands, here called the c-X band system, fall on bands of the B-2 system but fortunately a number of them are free from overlapping. The bands have the same type of rotational structure as the 2-x bands of ND3 and have been analyzed in the same way. Though the line width in the c-x bands is considerably greater than in the &-z bands, the analysis is quite definite and there is no doubt that they are parallel bands of NH3.The wave- numbers of the lines of the five bands which have been analyzed are given in table 3 and the constants which have been derived from the analysis are given in table 5. Six bands of the corresponding system of ND3 have been measured and analyzed. Here the lines are much sharper. The wave-number of the lines of the ND3 bands are given in table 4 and the constant2 derived from them in table 5. The bands are very similar in appearance to the A-2 band shown in fig. lb. The bands at the long wavelength end of the progressions are overlapped by the stronger &-z bands and experimentally it is impossible to determine the first member of the progression. Thus, there is no direct way of determining the vibra- tional numbering of the upper state vibrational levels.The NH3-ND3 isotope shift is not useful in this respect and the shift is so large that it is not possible to deter- mine which band of ND3 corresponds to a given band of NH3.a b FIG. 1 .-The 0-1 (a) and the 1-0 (6) bands of the 2-2 system. Note that the relative intensities of the R(O), R(1) and R(2) lines are quite different in the two bands. [To face page 162.a b / \ K j l 66 '6 *8 78 *2 48 '6 FIG. 2.-Portions of the 3-0 band of the fi-2 system with (a) and without (b) a magnetic field of 58,000 oersteds.A. E. DOUGLAS 163 170 = 65654.9 cm-1 vo = 66623.6 cm-1 65635.1 584.3 520.0 683.5 698.2 61 1.9 553.9 651.6 693-0 587.0 5564 646.5 635.1 695-5 559.9 525.6 487.8 639.4 624.8 66579.5 5 17.0 437.0 638.9 655-2 579.5 55 1.3 620.1 610.9 650.6 555.5 520.4 614.9 600.8 527.8 489.8 447.5 607-4 588.7 * Much of this band is overlapped by a band of the k-2 system.-f The K structure of each J line is not resolved. The K values listed are those which contribute substantially to the intensity of the line.I64 STATES OF NH3 TABLE 3-contd. v (cm-1) 704.2 709.3 630-2 61 1-9 618-9 596.9 605-3 579.6 589.7 572.1 553.9 Y (cm-1) 566.0 668.4 597.9 547.5 586.6 572.9 540.1 505-1 557.4 540.1 520.4 vo = 67608.9 * v (cm-1) branch (J)K 67604.8 QW 599.4 Q(W 591.7 Q(3)3 582.2 Q(4)4 570.0 Q(515 570.0 Q(413 557-0 Q(6)6 541.8 Q(7)7 532.7 P(3)OJ 524.1 Q(QS 5 12.3 P(4)3 504.4 Q(919 498.2 P(4) 1 * This band is overlapped by a band of CO. The bands of both NH3 and ND3 show a characteristic alternation in that alter- nate bands have strong R(0) lines.As with the 2-3 bands this characteristic is very useful in determining the vibronic symmetry of the excited state levels. TABLE 4.-wAVE-NUMBERS OF THE LINES OF THE c--? BANDS OF ND3 vo = 63581-67 vz = 64301.52 Y branch(J) K branch(J)K 63 571 -50 547.90 520.98 490.16 599.54 613.39 560.1 1 534.69 505-33 580.65 599.54 606.54 618.96 578-56 606.54 64279.69 267.07 253.17 238.16 221.78 204.72 18 6.03 3 18.36 324.97 330.66 334.75 300-32 298.09 294.19 294.79A . E. DOUGLAS TABLE 4-cantd. 165 575.38 571.50 565-60 559.27 551.65 543.21 533.44 65020.94 6499 6.0 1 64965.01 64928.40 65047.37 65057.38 65009.39 64996-0 1 6498 1 -37 64965.01 64947.98 64928.40 65030.02 65026.70 65047.3 7 65053.31 65057.38 64996.89 6498244 64965.68 64947-98 64928.40 65027.72 65023.1 6 65053-98 65057.38 64983.22 64966.96 64949.1 5 66746- 18 71 7.07 776-98 V 289.8 1 283-62 290.53 284.27 285-17 277-77 278.75 269.99 27 1 *27 261-28 262.79 251.57 253.17 240.83 242.72 231.13 YO = 65031.31 64929.93 65 024.43 65018-30 65010.72 65001.68 65059.36 64968-73 64950.78 6493 1.58 6 5020.0 8 650 12-44 65003 a4 1 650 14.74 65005.56 64995.03 64972.1 5 65008.36 64997.73 6498 5-53 65001 -01 64988.80 64975.21 64992.70 64978.85 64983.22 64968-06 64972.79 6496 1 *45 63949.15 vo = 65768.54 cm-1 65705.70 P(5)4 686.73 P(Q4 757.16 Q(4)4166 STATES OF NH3 TABLE 4-contd.789-01 746.63 7 17.07 699.97 767.18 789.01 733-96 7 1 8-49 70 1 -26 682.23 764.79 752.60 793.69 794-25 794.25 792.46 720-47 703.00 683.97 663.42 761.57 75425 735.15 795.43 796.9 1 796.9 1 738.35 72410 800.9 1 793.69 730.13 714-10 696.23 72 1 -02 703.09 683.97 710.79 699.72 66474.87 440.39 526-28 532.88 531-10 52 1 -49 489.84 475-41 459.18 440-91 510.33 506.28 526-9 1 531.10 748.33 737-78 725.44 799.51 799.5 1 796.91 690.57 669-32 751.89 741.25 728-92 714-82 802.89 800.90 796.91 790.80 67367 650.97 745.62 733-05 719.00 805.05 800.9 1 795.43 656-09 632.18 vo = 66511.83 6 6 5 2 9 * 3 5 463.4 1 445.05 504.56 496.47 48632 537.63 537.63 535-71 533.27 448.90 500.26 490.1 2 478.1 3A .E . DOUGLAS 167 TABLE 4-contd. 533.27 533.27 531-60 527.66 521.70 477-07 460.76 442.42 507.93 501.72 493-80 532.88 534.87 53487 533.27 67285.2 275.0 268.6 259.0 256.6 253.3 248.9 243.9 2376 230.2 541.54 539.48 535.71 433.39 495-00 482.69 468.69 544.49 540.44 488.82 474.87 458.39 546.46 48 1 -69 465.25 547.44 473.65 455-10 464-65 455.10 YO = 67260054 TABLE 5.-cONSTANTS DERIVED FROM THE c-? BANDS OF AMMONIA NH3 vt-v*t T;+G'(v2) B' C' vibronic transition (n+4)-0 67 60 8 -9 7-10 5.19 AS-A; (n+3)-0 6 6 6 2 4 - 2 7.615 5.166 Ai-A'; (n + 2) - 0 65654.9 8.1 1 5.12 AS-Ai (n+ 1)-0 64700.4 8.66 5-06 A1-A; n-0 63771.1 9-17 5.03 A"-A 1 N D 3 (n+ 6)- 0 67260.54 4.005 2.67 1 AZ-A; (n+5)-0 6651 1.83 4-122 2.645 Ai-A," (n+ 4) - 0 65768.59 4.246 2.626 A;I--A' (n+2)-0 64301.52 4-517 2.587 A;-A, (n+3)-0 6503 1-33 4.374 2.604 A;-A! (n+ 1)-0 63 5 8 1 *67 4.65 2.573 Ai-4,168 STATES OF NH3 SPECTRUM OF 15NH3 Because the isotope shift of ND3 bands with respect to those of NH3 is so large that the relationship between the two sets of bands cannot be established, the spectrum of 15NH3 was investigated. Here the isotope shift is small and there is no doubt which pairs of bands correspond.Since the rotational structures of the 14NH3 and 15NH3 bands are nearly identical, the isotope shift could be determined by measuring the relative wavelengths of a few corresponding lines in each band. The isotope shifts in the positions of a number of bands of the B-2 and C-2 systems, determined in this way are given in table 6. Once the 15NH3 isotope shift has been established, it was possible to find the correlation between the NH3 and ND3 bands. The NH3-ND3 isotope shift is also given in table 6. TABLE 6.-ISOTOPE SHIFTS OF THE 15NH3 AND ND3 BANDS OF AMMOMA band system 98-9’8 vO(NHSh’O(ND3) v o ( m 3 ) - l ’ 0 ( ’ ~ ~ 3 ) A-X €3-X 0-0 - 6-0 5-0 2 - 0 1 4 0 4 * (n+3)-0 (n+2)-0 (n+ 1)-0 (n- 1)-0 (n-2)-o* c-x n-0 584 cm-1 33-1 28.4 11.5 5.9 o*o* 1592.9 1353.4 11 18.7 896* 683* 488* 35.2 30.4 24.7 19.5* 14.3 * 9*0* These values have been obtained by extrapolating the observed values.OTHER BAND SYSTEMS All bands of NH3 and ND3 lying below 1400 A are so diffuse that even the sharpest of them show no well-resolved rotational structure. It appears that little definite information can be obtained from high resolution spectra and therefore spectra in this region have not been investigated in detail. No triplet states of NH3 are known in spite of the fact that the lowest electronically excited state probably is a triplet state.In an attempt to find absorption to the lowest triplet state, the absorption spectra of NH3 and ND3 were examined in a 5-m multiple reflection cell with absorption paths up to 150 m. The spectrum was examined from 3500A down to the A-2 absorption system and with ammonia pressures up to 1 atm. A number of absorption bands arising from impurities were found but none were found which could, with any certainty, be attributed to the triplet state of ammonia. ELECTRONIC STATES OF AMMONIA The symmetries and statistical weights of vibronic and rotational levels of NH3 and ND3 in the ground state and in excited states of D3h symmetry have been dis- cussed in previous papers.19299 In the following discussion, the symmetric and antisymmetric inversion levels of the ground state of ammonia will be designated by the symbols A; and A,”.For our present purposes, it is important to note that A i - 4 ; vibronic transitions in NH3 will have only odd J lines present in the K” = 0A. E. DOUGLAS 169 sub-band, whereas A;-Ai transitions will have only even lines present. In ND3, a ten-to-one statistical weight alternation in the K = 0 levels results in a similar effect but here the A;--A; transitions have strong odd numbered lines and A,"-A; strong even lines. In the parallel bands which result from these types of transition, the effect can be seen most readily in the R(0) lines. In the 0-0 band of the 2-2 system of ND3, the R(0) line is strong, thus showing that the upper state is vibronically A," and therefore since this is the u" = 0 level, 2 2 - P - 0 g4S FIG.3.-A schematic diagram showing the potential energy curves of NH3. D3h symmetry is assumed throughout since all known excited states of NH3 are of D3h symmetry and the energy required to force the ground state into this symmetry is only 3OOOcm-1. Other singlet states are noted in parentheses. electronically A;. The 1 4 band has a very weak R(0) line from which it follows that the upper state is vibronically A;. This A; vibronic level can arise only from the excitation of a vibration of A," symmetry and this can only be the out-of-plane vibration of a-planar ammonia molecule. Thus, the rotational structures of the bands of the A-;i7? system, in complete agreement with the conclusions of Walsh and Warsop,l show that the upper state is electronically of A; symmetry and that excitation of the v2 vibration gives the long progression of-bands.The diffuse Q head of the 0-0 band of the NH3, 2-X system was found to lie at 46160 cm-1, a value different from and probably more accurate than that given by previous observers. The band origin of the ND3 0 4 band is at 4671343 cm-1.170 STATES OF NH3 The frequency of the 0-0 band of ND3 is 554 cm-1 higher than that of NH3 thus implying that the vibrational frequencies of the upper state are much lower than those of the ground state. If it is assumed that v1 = v3 = 2v4 in both the upper and lower state then the obsezed shift of the 0-0 band corresponds to a value of about 2550 cm-1 for v1 in the A state of NH3 and 1810 cm-1 in ND3.Frequencies cal- culated in this way are quite inaccurate and the observed band of ND3 at 48908 cm-1 may correspond to excitation of v1 = 2194 cm-1 in the excited state. The present work on the &-x system has established two points. First, the 15NH3 isotope shift for the 0-4 band is very small and therefore the vibrational frequencies in the excited state must be approximately the same as in the ground state. Secondly, the observation of a Zeeman effect with a g value of 0.6, establishes that the state is a degenerate electronic state. The high value of g suggests that the e type orbitals responsible for the degeneracy of this state play little part in the N-H bonds. Prior to considering the C-x bands it is useful to review the states expected for ammonia from simple theory.In ammonia, as in all other hydrides, the states derived from the united atom concept should closely resemble the observed states. Thus, one useful method of determining the states of ammonia is to split the states of neon by a field of D3a symmetry. A second method of determining these states is to build up the molecular orbitals from those of N and €3. This has been done by Mulliken 10 and by Walsh 11 who have shown that the electron configuration of the ground state is (ls~)2(a;)2(e')4(a2")2. Orbitals of these symmetries can be derived from the united atom but since the e' and a; orbitals, both of which are derived from the 2p atomic orbital of the united atom, may be very different in energy, a notation which does not involve the united atom is desirable for the ground state. In orbitals with higher principal quantum number, the energy difference between molecular orbitals derived from the same atomic orbital may be expected to be small.This suggests that the orbitals of ammonia can best be written in the following order: ( ls~)(a;)(e')(a~(3sa;)(3pe')(3pa~(4sa;) . . . where the notation implies that (3saf)(3pe') . . . are similar to atomic orbitals. Thus the first few states of ammonia are expected to be : * (1 s ~ ) ~ ( a 1,)2(e1)4(a i), 'A;(C,,) = 3, (1) (2) (1sN)2(a ;)2(e')4(a 2")(3pe') 'E" = 8, (3) ( l ~ ~ ) ~ ( a 1 , ) ~ ( e ' ) ~ ( a i ) ( 3 p a ~ ) ' A ; , (4) (1 qJ2( a ;)2 ( e')3 (a g)2 (3sa 1,) ( 5 ) 1 If - (.1s~)~(a;)~(e')~(a;)(3sa;) A, - 2, E '. Fig. 3 gives a schematic potential energy diagram for ammonia correlating the states of the separated atoms and the united atom.The correlation between the observed and predicted states is satisfactory for the 3, 2 and f3 states. The ground state is, in D3h notation, of 1A; symmetry as pre- dicted. Also, as predicted, the first singlet excited state is of A; symmetry and there seems no doubt that the A state is the A: state resulting from configuration (2)- The state which is certainly a degenerate state could be correlated with either configuration (3) or (5) but the evidence is strongly in favour of (3). If the vibrational analysis given by Douglas and Hollas is correct then the state is E", not E', and * Walsh and Warsop 1 have described an additional state (lsr)2(a;)Z(e34(u~)(a;). There appears to be no way that such an additional state can arise from the united atom and here it is assumed that this state is a different description of the state (2) given above.A.E. DOUGLAS 171 must be associated with (3). Also the high g value observed here could be expected from configuration (3) whereas the e type orbital excited in configuration (5) is strongly associated with the bonding and the state should exhibit a strong Jahn-Teller effect and a corresponding low g value.12 state is not certain. It seems reasonable to associate the state with the 1A; state of configuration (4). Since the 3p electron plays little part in the bonding, this 1A; state should be similar to the state. Indeed, the c and s" states are similar in their B values, in their large cc2 values and in their 0 2 values.There is, however, as discussed below, a major difficulty to this assignment of the c state. The vibrational numbering of the bands of the c-2 system cannot be deter- mined directly from the observed spectra. Alternate bands of this system do have strong R(0) lines, thus indicating that the excited state is planar or nearly so. Since, except for the effects of zero point vibration, B = 2C in the lowest vibrational level of a planar symmetric top we can determine the vibrational numbering by finding the level wherein this condition is fulfilled. This level is not observed directly but can be determined by extrapolation. The large value of cc2 makes the extrapolation reasonably accurate. The extrapolated values of VO, B and C for NH3 and ND3 are given in table 7.Here we see that the level of NH3 which comes closest to fulfilling the relationship B = 2C, is the vibronically A; level at 62010 cm-1 and that this level has a NH3-ND3 isotope shift of 488 cm-1. This may be compared with the state where, from 15NH3, we find the isotope shift is zero in the level where B = 2C. If the vibrational level of NH3 at 61207 cm-1 is considered to be the zero level then it is vibronically A; but the relationship B = 2C is less well fulfilled and there is still an isotope shift of 320 cm-1. This isotope shift would correspond to v1 having the extremely high value of -4000 cm-1 in the upper state (assuming 0 1 = 0 3 = 204). Thus the assignment of the state to electron configuration (4) leads to the unlikely result that the 3pa; orbital has quite different bonding prop- erties than the 3pe' orbital.The assignment of the TABLE 7.-EXTRAPOLATED VALUjS FOR THE CONSTANTS OF CORRESPONDING LEVELS OF THE C STATE OF NH3 AND N D 3 M I 3 ND3 To+ C ( U 2 ) B c To+ G(02) B C ,,, vibronic symmetry (n- l)-Ai 62870 9.67 5-00 62186 4.91 2.56 (n- 2)- A; 62010 10.18 4.97 61522 5.04 2-55 (n- 3) - A 61207 10.68 4.94 60887 5.17 2.53 (n- 4)- A," 60476 11.2 4.91 60289 5.30 2.52 (n- 5)- A ; 59825 11.7 4.9 59738 5.4 2.51 As an alternative assignment, the c-x bands may be attributed to a vibrational progression of the B-X system. If one quantum of a degenerate vibration of the B' state is excited E", A;' and A; vibronic levels result. The E" and A; vibronic levels have symmetry species which can combine with the ground state but unless the intensity of these transitions is enhanced by some interaction, such as Fermi resonance or strong vibrational-electronic interaction, we should expect the transi- tion probability to be low. Considering this possibility and assuming the 62010 cm-1 band of NH3 is the A: band resulting from the excited E vibration, we find values of 2786 cm-1 and -2300 cm-1 for the frequency of the degenerate vibration in NH3 and ND3 respectively. The ratio of these two frequencies is far from that expected from the normal isotope effect and even further from the ratio expected in a molecull172 STATES OF NH3 exhibiting a Jahn-Teller effect.Also these frequencies are much lower than the corresponding frequencies in the ground state and thus are incompatible with the observation that the isotope shift of the 0 4 band is extremely small.Therefore it is most unlikely that the c-x bands are a progression of the &-x system. Probably the most likely explanation of the state is that it is the 1A; state of electron configuration (4) above, but that its B value is affected by Coriolis inter- action of the B' and states. The interaction of a degenerate and a non-degenerate vibrational level of the same electronic state has been treated by Garing, Nielsen and Ra0.13 This interaction may lead to a " giant Z-type doubling " in the K = 1 levels of the degenerate state and a related change in the B value of the non-de- generate state. The interaction of an electronic A and E state should have a similar effect. A large Z-type doubling has been observed in the B-state and it is quite possible that it arises from the interaction of the c and B states.If the NH3 level at 59825 cm-1 (table 7) is considered to be the zero level of the C state this electronic state lies a few hundred cm-1 above the state. This arrangement of levels would, in the notation of Garing, Nielsen and Rao lead to a negative value of a'21la and an effective B value of the state greater than the true value." Thus, this shift of vibrational numbering will give an effective Bo much greater than 2C0 but the true Bo may still equal 2Co. No quantitative agreement can be expected between the perturbation in the c and B states since the a' value of the degenerate state can be the sum of contributions from a number of sources.13~ 14 PREDISSOCIATION All the excited states of ammonia are predissociated. The predissociation is least noticeable in the B-2 bands where the lines of NH3 are about 0-4 cm-1 wide and those of ND3 are too narrow to be measured. The effects of predissociation are greater in the c--2 bands where the lines of the NH3 bands are about 1.0 cm-1 wide and again those of ND3 are much narrower.All lines within a band of the B--Z and C-2 systems show approximately the same width and the line width does not vary noticeably from band to band in the progressions. The predissociation of the 2 state is more complex. This predissociation can best be studied in the ND3 bands where the lines are much narrower than in NH3. The bands of ND3 with I& = 0 as their upper state are diffuse, having line widths of about 2.5 cm-1 while the lines in bands with ZI; = 1 are sharper with line widths of about 0.8 cm-1.All higher levels are much more diffuse. All bands of NH3 are too diffuse to show measurable rotational structure. The = 0 and ZJ; = 1 levels have about the same band width but there is a slow increase in band width with increasing 2~2 beyond the second level. Near 1600A there appears to be a weak continuum which may arise from higher levels of the 2-x system. Aside from the peculiar variation in the degree of predissociation with 212, it is difficult to understand how the 2 state predissociates. It is known that the absorp- tion of light into the 2-x bands leads to the reaction N H ~ + ~ V - - + N H ~ + H .~ ~ ~ 16 The potential energy curves for a hydrogen atom approaching a NH2 molecule to form NH3 are shown in fig. 4. Here there are only two curves lower than that of the A; state which could cause the predissociation. One is the ground state curve and it appears unlikely that two states so far removed from each other will interact strongly. The other lower curve is that of the unobserved 3A; state. Here also it appears unlikely that the interaction between states of different multiplicity would be strong enough to give the observed effect. In addition, the 3 4 state might be * Douglas and Hollas have assumed q = ct'Z/u to be positive but their analysis does not deter- mine the sign of this quantity. If q is negative the B values given by them must be increased by q.A .E. DOUGLAS 173 expected to follow a curve similar to that of the 1Al state and thus in itself be a stable state. state is sufficiently shallow to allow predissociation through the potential barrier without interaction with another state. There is some information on the depth of this curve. As pointed out earlier, the N-H stretching frequencies in the state are probably about 2600cm-1. Though this value is much lower than in the ground state it is still sufficiently high to indicate that there is a marked potential minimum and escape of a hydrogen atom through the potential barrier from the u = 0 level seems most unlikely. It may be assumed that the potential curve of the Bl I 2 -BI -2A, 2 - Bi r(H-NH2) FIG. 4.-A schematic diagram of the potential energy curve of a hydrogen atom approaching a NH2 molecule along its CzV axis.The peculiar shape of the A; curve is assumed to arise from the interaction of the two dashed curves. Only the lowest triplet state (a state which has not been observed) is shown as a dotted curve. There seems only two mechanisms through which the predissociation can occur. First, we can assume that the singlet-triplet interaction is strong and that the triplet state is unstable. Secondly, we may assume that the potential curve of the excited state is far from that expected for a state having simple valence forces. Thus, the potential curve given in fig. 4 may be valid for a normal NH;! molecule but for small changes in the NH2 bond distances and angles the barrier to predissociation may be much smaller. In this way certain relative phases of the two components of the degenerate stretching vibration in the molecule may lead to dissociation. This escape1 74 STATES OF NH3 route does not necessarily alter the vibrational frequencies very greatly. * There is no positive evidence that such a potential actually exists but the observations are not inconsistent with its existence. I wish to thank Dr. G. Herzberg for numerous suggestions during the course of this work and Mr. F. Alberti who photographed all the spectra used here. * Prof. H. C. Longuet-Higgins originated this concept and has developed it further by showing that a two-dimensional oscillator in a potential well entered by three symmetric valleys (i.e., an ash- tray potential) has its vibrational frequencies altered little by these valleys. 1 Walsh and Warsop, Trans. Faraday SOC., 1961, 57, 345. 2 Douglas and Hollas, Can. J. Physics, 1961, 39, 479. 3 Douglas and Potter, Appl. Optics, 1962, 1, 727. 4 Douglas, Can. J. Plzysics, 1958, 36, 147. 5 Benedict, Physic. Reu., 1935, 47, 641. 6 Benedict and Plyler, Can. J. Physics, 1957, 35, 1235. 7 Rao, Brun, Hoffman, Jones and McDowell, J. Mol. Spectr., 1961, 7, 362. 8 Crawford, Rev. Mod. Physics, 1934, 6, 90. 9 Dressler, J. Chem. Physics, 1960, 32, 1682. 10 Mulliken, J. Chem. Physics, 1935, 3, 506. 11 Walsh, J. Chem. SOC., 1953, 2296. 12 Strauss and Coulson, Proc. Roy. SOC. A , 1962, 269, 443. 13 Casing, Nielsen and Rao, J. i’dol. Specfr., 1959, 3, 495. 14 Child, Mol. Physics, 1962, 5, 391. 15 Dressler and Ramsay, Phil. Trans. A, 1959, 251, 453. 16 McNesby, Tanaka and Okabe, J. Chern. Physics, 1962,36, 605.

ISSN:0366-9033    

DOI:10.1039/DF9633500158

出版商:RSC    

年代:1963

数据来源: RSC

摘要:

The 3820 A Band System of Propyiial BY J. C . D. BRAND, J. H. CALLOMON AND J. K. G. WATSON The University, Glasgow, W.2, and University College, London, W.C. 1 Received 2nd January, 1962 The first singlet-singlet r * - n transition of gaseous propynal, C2H. CHO, gives rise to a band- system in absorption centred near 3820 A. The rotational and vibrational structures of this spectrum, together with that of the deuterated isotopes C2H. CDO and C2D. CHO, have been extensively analyzed. Of the 12 excited state fundamental frequencies, all but one have been assigned and measured. Rotational analysis reveals a small positive inertial defect in the excited state, as in the (planar) ground state, and it is concluded that the excited state is also planar. All non-totally sym- metric vibrations appear in the spectrum, but only in very short progressions, and this is attributed to " intensity stealing ".Several of the numerous Fermi and Coriolis couplings that are observed have been analyzed. The excited-state vibration21 frequencies show that the effects of electronic excitation are not limited to the carbonyl chromophore, but also transmitted to the furthest atoms in the acetylenic chain. In propynal vapour a weak absorption system extends from about 3820A to- wards shorter waves.19 2 The bands mark a transition analogous to the one-electron rc* --n excitation of formaldehyde, which leads there to a non-planar excited state with an out-of-plane angle estimated to lie between 20" and 2 7 O . 3 ~ 4 It seemed to us of interest to determine whether similar geometrical changes accompany the transition in propynal, where conjugation might be expected to favour a more nearly planar configuration.The structure of propynal is generally favourable for rotational analysis of the ultra-violet bands. As the molecule is very slightly asymmetric ( K = -0.9897 in the ground state 5) only those levels with K = 0 or 1 are displaced appreciably FIG. 1. from the symmetric top values; and the configuration is strongly prolate so that the sub-band origins, except where the degradation produces a head, are well separ- ated from one another. The orientation of inertial axes in the ground state, which is planar, is shown in fig. 1. Since the bands appeared sharp to about 3000 A under low dispersion the prospects for a vibrational analysis appeared good.In the event the detail that could be established exceeded all expectation. 175176 SPECTRUM OF PROPYNAL EXPERIMENTAL The spectra of the normal isotope and of two singly-deuterated derivatives, C2D. CHO and C2H. CDO, were taken in the second order of a 6 m Ebert grating spectrometer described by King.6 At long wavelengths the spectrum is exceedingly sharp, the width of individual rotational lines being less than the instrumental resolution of the grating (ca. 0.09cm-1); but at shorter waves the structure becomes increasingly diffuse, the diffuseness being more pronounced for C2D. CHO than for C2H. CDO or the normal isotope. A number of low-resolution plates were taken with the vapour heated to 200” to determine the effect of temperature on the various bands.The f-value of the system is - 5 x 10-4, roughly twice the intensity of the corresponding transition in formaldehyde ( f = 2 . 4 ~ 10--4).7 RESULTS AND DISCUSSION ROTATIONAL STRUCTURE The principal bands in the spectrum are perpendicular bands of a near-symmetric top. How much detail is visible depends, among other things, on intensity and freedom from overlapping, but in favourable conditions it was immediately obvious that the features to the red of the band centre corresponded to the P-type P- and Q-branches of a perpendicular band. In well-developed sub-bands the PP-branches could be followed for thirty or forty members, so that values of J and K could be assigned unambiguously from the number of missing lines in each branch.2 As the J-structure degrades very slowly to the violet the @branches are prominent, like the Q-branches of an infra-red band.Once the P-type sub-band origins were established the A-type origins could be identified from combination differences of the form, A2F”(K) = RQK-l-PQK+I = 4(Ag-B;;)K-8DiK(K2+1) using microwave values for the constants A: and z;.5 Since the K-structure de- grades rather sharply towards the red the R-type sub-bands form a head of Q-branches at quite low values of K. Fig 2 shows the central region of the 3821 A band (the electronic origin) of C2H . CHO. From the constants A0 and Bo and the assumption of a small positive inertial defect it is possible to calculate the position of lines in the Q-branches of low K. It then emerges that the accumulation of intensity near the band origin is accounted for if, and only if, the transition moment responsible for this band coincides with the axis of greatest inertia (type C band).This result might have been obtained intuitively for the very slight degradation in the J-structure implies that the band centre will have a contour similar to an infra-red band, so that the high intensity about the origin has essentially the same explanation as the central maximum of a type C vibration-rotation band. Although all the strong bands in the spectrum (including the electronic origin) and most of the weaker ones are type C there are a few weak bands of different con- struction. Of them, some are perpendicular bands with a central minimum and can be explained along the lines of fig.2 as due to hybrid transitions polarized almost parallel to the inertial b-axis, while others are parallel bands, type A (AK = 0), the transition- moment lying nearly along the a-axis. A very few bands are A-B hybrids with the intensity divided roughly equally between the perpendicular and parallel components. Rotational constants relating to the vibrationless levels of the combining states in the light isotope are set out in table 1. The lower state K-structure constant (A; - 5;) agrees well with the more accurate values calculated from the microwave spectrum,s although the centrifugal constant D&, which could not be separately determined by microwaves, was there included in the effective principal constant A,”.€30. 2.-The electronic origin band of C2H.CHO, with Fortrat curves for Q-branches near the band-centre. Length of cell, 1 m ; pressure [To face page 176. of gas, 5 mm.J . C. D. BRAND, J . H. CALLOMON A N D J . K . G. WATSON 177 TABLE 1 .-SYSTEM ORIGIN AND ZERO-POINT ROTATIONAL CONSTANTS (C2H. CHO) Too 261 62.89 GROUND STATE 2.1 1387 0.1 55538 2 . 9 0 ~ 10-4 - 0.9897 +0.1718 a.m.u. .@ EXCITED STATE 1.7340 0.1 5660 f0.00001 1.8906 0.1 6336 f0.00002 0.14987 f0.00002 2.73 x 10-4 - 0.9844 (4 +Om40 f0.04 a.m.u. & (f l (a) AU units cm-1, except those labelled (e) and 0; (c) = +(I?+ C) ; ( d ) after correcting the effective microwave (= Ao- B,) ; (e) asymmetry-parameter (dimensionless) ; (b) From the microwave spectrum; (f) inertial defect (&--la-&), a.m.u. A2. ELECTRONIC A N D VIBRATIONAL ASSIGNMENTS The isotope shifts and the assignment of hot bands in the spectrum show that the strong type C band near 3821 A (see fig.2) is definitely at the electronic origin. As the rotational analysis contains no evidence of electronic angular momentum in the excited state the results collectively indicate that the transition is 1A’’-lA’ in nomenclature appropriate to planar configurations, consistent with its interpretation as a one-electron 71”-n transition. A number of aids were used in order to extract the vibrational frequencies from the spectrum. Apart from isotope shifts and the effect of temperature on intensity these aids were mostly concerned with the presence or otherwise of regular perturba- tions. In all the stronger bands the K-structure provided at least several values of the initial and final state combination differences, AzF”(K) and A2F’(K), which were evaluated as fully as the band development allowed.When there is an appreciable component of Coriolis coupling along the a-axis in either state the vibrational angular momentum is reflected in the A2F(K), whose values may differ sufficiently from expectation to characterize the state in question. As an example, we give in table 2 a few results for two bands close to the electronic origin. Both bands are less intense at 200” than at 20” and so must emanate from the vibration- less ground state or from a vibrational level close to the ground state. The first possibility is ruled out by the fact that the A2F”(K) are different from those observed for the ground state, thus both bands must be hot bands: in fact, the A2F”(K) are symmetrically disposed above and below the ground state values and hence the initial states are mutually coupled by Coriolis forces.The only two ground state vibrational levels which meet this specification are vF and v1;’ so that the initial state of each transition can be identified. Moreover, analysis of the coupling gives the constant I [G,i2 I from which one may calculate approximate values of the A2F”(K) for transitions emanating from the overtone and combination levels 2v&178 SPECTRUM OF PROPYNAL vF+v;; and 2vy2, which do give rise to observable transitions. Effectively, the vg, vy2 coupling serves as a fingerprint. The form of analysis is equally successful for certain pairs of vibrations which are coupled in the excited electronic state.TABLE 2.-GROUND STATE K-STRUCTURE COMBINATION DIFFERENCES (C2H. CHO) A2F”(K) (cm-1) = “ Q K - ~ - ~ Q K + ~ for : K vy2 = 1 v; = 1 zero-point mean of col. 2 and 3 3 4 5 6 7 8 9 10 27-3 23.6 36.1 31.5 44.9 39.5 53-5 47.4 62.1 55.3 63.2 71.2 78-9 I 55.72 1 = 0.639 25-38 25.4 33.61 33.8 41-98 42.2 50.27 50-4 58-43 58.7 66-41 74.39 82-14 Another criterion is connected with the anharmonic resonances which are wide- spread in the A” state. For instance, Fermi resonance between v i and v;+t$ is seen in the spectra of C2H. CHO and C2D . CHO and its effects extend to all over- tones and combinations containing vi. Since vi is the most active excited state vibration the resonance allows a large number of levels to be recognized easily as combinations including v i : while, conversely, any level higher than v; but not TABLE 3.-EXAMPLE OF FERMI RESONANCE : Vi-Vi-Vi IN C2H.CDO Assumed : unperturbed GO(V2,04,t’6) = C W f U j + C X i V i V i ; V (Fermi) = hC.kz46qzq4q6; the q’ are dimensionless normal co-ordinates, and k246 is a cubic force-constant, in cm-1. i=2,4,6 i,j=2,4,6 level calc. obs. unperturbed perturbed v4 1267.4 - 1267-4 2v4 25 19.6 - 25 19.4 3v4 3756-6 - 3756.8 v6 941 -9 - 941-9 2v6 1878.5 - 1878.5 2204.0 2 190.4 2 1 89-8 2208.0 2221 -6 2221.3 3471.0 3487-6 3488-6 3458.7 34421 3442.9 4722-6 4738.9 4738-0 469443 4678.5 4678.5 3 130.8 31 14.3 3115.1 3143-3 3 159.8 3 159-0 4330.0 4322.2 4322-5 43963 4428.2 4427-2 4392.7 4368-8 4369.5 w$ = 1275-0, wg = 94455cm-1, ~ 4 4 = -7.6, x66 = -265, x26 = -15.1, x46 = -1.3, I k246 1 = 43.7.J .C . D . BRAND, J . H . CALLOMON AND J . K . G. WATSON 179 affected by resonance is either a fundamental or a combination not involving v;. Thus a band at 0 + 1945.5 cm-1 (0 + 1850.0 and 0 + 1945-8 cm-1 for C2D . CHO and C2H. CDO, respectively) must be associated with an upper-state fundamental, and its isotope shift then identifies it with the C r C stretching mode v;. Table 3 gives a representative analysis of the v;, vi-tv; Fermi resonance in the spectrum of C2H. CDO. Table 4 lists the assigned fundamentals in the ground and excited states. TABLE 4.-FUNDAMENTAL FREQUENCIES OF PROPYNAL approximate description CHZ stretch CHI stretch CzC stretch C=O stretch CHI rock C-C stretch L CCH bend L CCO bend L C-C E C bend IA’ (ground) state 1A” (excited) state C2H .CHO C2D. CHO C2H. CDO C2H . CI-IO C2D . CHO C2H . CDO 3326 2858.2 2106 1696.9 1389 943.7 650.0 613.7 205.3 2605 2858.6 1977 1697.0 1387.6 933.6 507.9 609.0 195.6 3326 2118 2101 1679 6 1080.0 876.5 649.7 61 1.9 201.5 C2H. CHO C2H. CHO C2D. CHO C2H. CDO product rule for a‘ fundamentals : ~~ obs. 1.861 1.936 - harmonic value 1 -892 1-937 - v1o(a”) CHI wag 981.2 980.9 841.0 4621 459.0 q 1 ( d ’ ) LCzCH bend 692.7 548.6 6913 389.7 291 vlz(a”) L C-Cr C bend 260-6 248-5 249-9 345.9 346.9 (1.5) (2.1) (4) (0) (3.0) (2.5) C2H. CHO C2H. CHO C2H. CHO C2H. CHO product rule for a” fundamentals : - _ _ _ ~ C2D . CHO C2H. CDO C2D . CHO C2H. CDO obs. 1.324 1.219 1.344 1 -255 harmonic value 1.336 1-21 8 1.331 1 -242 0 relative intensity of first quanta, origin band = 10.6 corrected for Fermi resonance ; c precise value uncertain. GROUND-STATE FUNDAMENTALS All ground state fundamentals below 2000cm-1 occur at least once in the electronic spectrum (as 0-1 or 1-1 transitions) though vI; is seen only for C2D. CHO and v;l only for C2H. CDO. The fundamental frequencies in table 4 differ from earlier infra-red measurements 8 in two respects. First, a re-examination of the 2100 cm-1 region with an infra-red spectromster of higher resolving power has prompted some revision of the v;’ and v; fundamentals of C2H. CDO, but the changes are small and comparatively unimportant. Secondly, however, the180 SPECTRUM OF PROPYNAL electronic spectrum requires the original assignments of the L C E CH in- and out- of-plane bending modes, v;(a') and vyl(a"), to be interchanged. These two funda- mentals are strongly coupled by Coriolis forces, and are seen in the infra-red as two overlapping perpendicular bands, the former predominantly type B, the latter wholly type C.An absorption-minimum near 688 cm-1 was previously ascribed to the band-centre of v7, type B, and a maximum at 661 cm-1 to v11, type C.8 On this assumption, the partially-resolved and weak K-structure on both wings of the ab- sorption could be numbered and analyzed to yield a value for the Coriolis coupling- constant. This simple interpretation neglected, however, the effect of the Coriolis coupling on the intensities of the bands, which can result in a strong asymmetry in intensity between AK = + 1 and AK = -1 sub-bands: in the limit in which the two coupled vibrations become merely two components of a degenerate mode of a prolate symmetric top or linear molecule, each component of the degeneracy provides only either the X-type or the P-type sub-bands, i.e., two half-bands having the appearance of a single whole perpendicular band.9 In the present case a strong Coriolis intensity-effect is undoubtedly present in the infra-red v7, v11 bands, making their appearance misleading.The correct assignment of vYl is deduced from four ultra-violet transitions having states in common, a fact which could be established from K-type combination- differences. Four vibrational levels are involved (fig. 3), two of which could be identified with the known via and vY2, also by combination-differences (table 2).FIG. 3.-Cross-sequences in the spectrum of C2H . CHO. The relative strengLs are obtainer visual estimates of the intensities corrected by Boltzmann factors. from Together with the electronic origin band, sufficient information is available to cal- culate the frequencies of sub-bands of v11 in the infra-red, and hence to renumber those resolved there. Similar arguments can be applied to the infra-red v7, and the coupled bands reanalyzed. The revised band-origins are at 692.2 cm-1 (vY1 : C2H . CHO) and 649.5 cm-1 (v; : C2H. CHO). The v;&") origin lies close to an intensity-maximum as expected, but the minimum to be expected for the centre of v"(a') lies on a steeply-falling part of the intensity-curve, and although recog- nizable, is not prominent.The magnitudes of the revised Coriolis constants are [;,yl (C2H . CHO) = 0.922 ; (C2D . CHO) = 0.892 ; and (C2H . CDO) = 0.915, close to the value of unity for degenerate vibrations of a linear molecule.J . C. D . BRAND, J . H. CALLOMON A N D J . K . G. WATSON 181 EXCITED-STATE FUNDAMENTALS Besides the excited state fundamental frequencies, table 4 lists the relative in- tensities (on a scale which accords the system origin an intensity of 10) with which the 1-0 transitions appear in the spectrum. The outstanding feature is the high intensity of the CO stretching fundamental v;, implying a large change in the CO bond distance on excitation. The fact that all the a’ fundamentals except vl are active suggests that geometrical changes are rather widespread; but with the ex- ception of vi the 2-0 transitions tend to be weak, so that most of the changes must be relatively small.There is evidence that some of the intensity associated with the 1 4 transitions in v; and vi is explained by the presence in the normal co-ordinate of the internal co-ordinate for CO bond stretching. Of interest is the occurrence, as mentioned above, of bands of all possible polar- ization types in the spectrum, and analysis identifies progressions in all three of the upper state fundamentals which correspond to the non-totally symmetric (out-of- plane, a”) modes in the planar ground-state. (The 1-0 transition in vil is exceed- ingly weak, but as vil occurs also in sequences its numerical value is not in question.) Two explanations are possible : (i) that the excited state is non-planar, i.e., retains no elements of symmetry, all fundamentals being strictly speaking totally sym- metric and hence potentially active in progressions according to the Franck-Condon principle ; (ii) that the non-totally symmetric vibrations a” perturb the planar excited-state electronic wave-function A” and mix it to some degree with another higher state A‘, thereby acquiring non-zero intensity in the spectrum by virtue of “ intensity-borrowing ” from transitions to this higher state.The shortness of the progressions and the small positive inertial defect of the excited state rule out serious departures from planarity ; potential maxima separating possible double potential minima about the molecular plane cannot, therefore, be high, and if present at all should be reflected in appreciable anharmonicities in the out-of-plane vibrations.Judged from these viewpoints the skeletal vibration vi2 is certainly active through intensity-borrowing, for it is seen in sequences to be harmonic up to three quanta, and the intensity of the 2-0 transition is of the order of magnitude to be expected from the ratio of frequencies vi2/vY2. The fundamental v;l is so weak as to argue against any change of out-of-plane angle at the terminal carbon atom in the CCH chain. The large ratio vi,/vyl implies that the 2 4 transition in v11 should have appreciable intensity, but no transition turns up at 0+2vil so that there could be some anharmonicity in the potential.vi0 is of importance in that any non-planarity in the CHO group, such as occurs in excited formaldehyde, would be detected from the behaviour of this fundamental. The 2 4 transition in v10 does appear in the spectrum though most of its intensity (I = 2.0 on the scale used in table 4) is explained by Fermi resonance with v;. After correction for the resonance the location of the 2-0 transition, at 0+949.0 cm-1, is about 25 cm-1 higher than the two-fold fundamental frequency (2 x 462.1 = 924.2) ; thus there is anharmonicity in the same sense, though not nearly so large, as observed with formaldehyde.3.4 The an- harmonicity in via, and the possible anharmonicity in vil, may explain why the pro- duct rule ratio for the excited state a’’ frequencies is in excess of the harmonic value.To see what the vi0 anharmonicity might mean in terms of non-planarity in the CHO group the two observed quanta have been fitted to an equation of the form, v = hcv[+q2 +a exp ( -pq2)] in which a Gaussian function is added to the ordinary harmonic potential.10 Here, v is the limiting harmonic wavenumber for large v, 4 is the (dimensionless) normal182 SPECTRUM OF PROPYNAL co-ordinate for the mode via, and a and p are parameters. The potential has a double minimum unless 2ap < 1. With only two quanta observed, several arbitrary values were assigned to p and the equations solved for the parameters v and a. It emerges that the potential does not necessarily have a double minimum in order to fit the observed results, though a double minimum is by no means ruled out.Assuming an effective mass of 1 a.m.u. the allowed range of values corresponds to an out-of-plane angle between the CH bond and the CCO plane of 0-4" : thus the potential function is probably rather flat for about 4" on either side of the planar configuration, possibly with a central maximum. An interesting feature of the spectrum is the occurrence of transitions which seemingly exchange quanta ofvll and v12. Both vibrations are active in n-n sequences of the usual type, but in addition we observe " false " sequences in which transitions from an initial state with, for example, vy2 = 1 lead to a final state with vil = 1. The assignments are established beyond doubt by combination differences and are shown schematically in fig.3. The explanation is that, whereas vYl and vY2 are essentially pure C=CH and CCC out-of-plane bending modes, the excited state vibrations are roughly equal mixtures of the two co-ordinates : therefore, transitions from the level vY2 = 1 may lead to the CCC bending component in either vi2 or v ; , with relative intensities determined by the coefficients for CCC bending in the normal co-ordinates for ui2 and vil. This mechanism does not seem to have been noticed previously but is probably to be found quite commonly in polyatomic spectra. ELECTRONIC STRUCTURE OF THE EXCITED STATE The dominant frequency changes on excitation are those affecting v4, v10, v11 and v12. The effect on v4 and v10 might have been expected by analogy with formal- dehyde ; but the change in v11, almost as great as that in v10, shows conclusively that the excitation is not localized in the CHO group.Using conventional valence formulae the results suggest that the excited state should be represented as a hybrid of /" H / \- \- H-C=C-C* and H-C-C-C 0- 0. with roughly equal weights for the two structures. The implied delocalization possible in propynal partly relieves the strain which forces formaldehyde out of plane, leaving excited propynal planar or very nearly so. Estimates of the change in bond distance can be made for those normal vibrations which are essentially group vibrations using Badger's rule or Clark's rule.11 Results for the CO, C r C and formyl CH bonds (table 5) are consistent for the three isotopes, suggesting that this simple analysis has some validity. From the relative intensity TABLE 5.-BOND DISTANCES (A) IN EXCITED PROPYNAL r"a r' (Clark) r' (Badger) r' (mean) bond 1 I1 111 I 11 111 C=O 1.215 1.327 1.328 1.334 1.318 1.319 1-325 1.325 G C 1.209 1.241 1.236 1.240 1.238 1.233 1.237 1.238 CHI 1.106 1.094 1-094 1.092 1.090 1.090 1.087 1.091 (fOrnY1) I = C2H.CHO I1 = C2D. CHO I11 = C2H. CDO = ref. (5).J . C . D. BRAND, J . H. CALLOMON AND J . K . G . WATSON 183 of the principal bands in solution (vi progression) the calculated increment in the CO bond distance is -0.10A,1 and a similar result is obtained from the vapour intensities : thus it appears very probable that the excited state CO bond distance is virtually the same as in excited formaldehyde (1-32 A).3,4 It is difficult at present to say anything about the changes in bond angle but a complete set of rotational constants will provide further structural information and this work is in progress. We thank D.S.I.R., the Carnegie Trust, and Esso Research Ltd., for financial support, and Dr. C . C. Costain for a sample of deuterated propynal. 1 Howe and Goldstein, J. Amer. Chem. Soc., 1958, 80,4846. 2 Brand, Callomon and Watson, Can. J. Physics, 1961, 39, 1508. 3 Brand, J. Chem. SOC., 1956, 858. 4 Robinson, Can. J. Physics, 1956, 34, 699. Robinson and Di Giorgio, Can. J. Chem., 1958, 5 Costain and Morton, J. Chem. Physics, 1959, 31, 389. 6 King, J. Sci. Instr., 1958, 35, 11. 7 Duncan and House, quoted by Pople and Sidman, J . Chem. Physics, 1957, 27, 1270. 8 Brand and Watson, Trans. Faruday SOC., 1960, 56, 1582. King and Moule, Spectrochim. 9 Teller and Tisza, 2. Physik, 1932, 73, 791. 36, 31. Acta, 1961, 17, 286. 10 Chan, Zinn and Gwinn, J. Chern. Physics, 1961, 34, 1319. 11 Herzberg, Spectra of Diatomic Molecules (Van Nostrand, New York, 2nd edn., 1950), p. 457.

ISSN:0366-9033    

DOI:10.1039/DF9633500175

出版商:RSC    

年代:1963

数据来源: RSC