J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3213-3219 Gas-phase IR Spectrum of 7-Azaindole Scaled Quantum Mechanical Force Field and Complete Spectrum Assignment Elisabetta Cane, Paolo Palmieri, Riccardo Tarroni and Agostino Trombetti" Dipartimento di Chimica Fisica ed lnorganica, Universita di Bologna, Wale Risorgimento 4,40136 Bologna, Italy The gas-phase IR spectrum of 7-azaindole has been recorded from 100 to 4000 cm-', using a multipass cell heated to ca. llO°C, and completely assigned using theoretical predictions based on the scaled quantum mechanical (SQM) method. The harmonic force field of 7-azaindole, evaluated at the HF-SCF level using 6-31G** orbitals, is corrected by scaling the force field over a convenient set of internal coordinates. Scaling factors were determined by least-squares fitting of the theoretical to the experimental frequencies of the two parent molecules, pyridine and pyrrole, and their perdeuteriated isotopomers.Our final prediction gives frequencies for 7-azaindole which on average differ from experiment by 18 cm-'. 7-Azaindole (7-A21), is an important bicyclic aza-aromatic molecule: it is iso-electronic to purine and has a close relationship with the nucleic bases adenine and guanine. Its planarity, in the gas phase, has been demonstrated by micro- wave spectroscopy;' additional spectral properties have been determined by electronic spectro~copy.~-~ By comparison, the information available on its vibrational properties is scarce, since only the Raman spectrum in methanol solution hGzeportedby Fuke et aL2 Using a multipass cell heated to ca.llO°C, we have recorded the gas-phase IR spectrum of this molecule, at low pressure, from 100 to 4000 cm-', and in the following sec- tions we assign all fundamentals. The main advantages of the IR gas-phase spectrum over the solution or solid-state spectra are that the symmetry of most vibrational coordi- nates is easily determined from the rotational profiles of the IR bands and that hydrogen-bond formation and other inter- molecular interactions are avoided. For the assignment of the IR bands, we compare the experimental and the theoretical IR spectrum evaluated by the SQM force field method of Pulay and co-workers.6-s We have implemented this method and have recently reported a similar investigation on the related molecule indazole,' of similar complexity ; a few applications to bicyclic molecules have also appeared in the literature."-12 Using Hartree-Fock theory and standard atomic orbital sets, the harmonic force field of the molecule has been first evaluated by ab initio methods.The main limitations of the theoretical description were next removed using empirical scaling factors for the force field, which were determined by fitting to the experiment theoretical IR spectra of simpler, closely related parent molecules evaluated at the same level of theory; pyridine and pyrrole were taken as the natural refer- ence molecules of 7-AZI. With the optimized scaling factors, we make our final prediction of the IR spectrum and, by comparison with the experimental spectrum, assign all funda- ment als.Experimental 7-AZI (mp 105-7°C) was purchased from Aldrich Chem. Co. (98%) and was purified by vacuum sublimation at 50 "C. The gas-phase IR spectrum was recorded on a Bomem DA8 Fourier-transform spectrometer from 100 to 4000 cm- ' using a multipass cell with a 4.8 m pathlength, heated to ca. 110 "C.The spectrum above 450 cm -was measured using a KBr beam splitter and a liquid-nitrogen-cooled MCT detec- tor, with a final resolution ranging from 0.060 to 0.180cm-'. The spectrum from 100 to 650 cm-'was recorded with 3 and 6 pm Mylar beam splitters and a DTGS detector; the sample was heated to ca. 110°C and the final resolution of the spectrum is 0.2 cm-'.At least 256 scans were accumu- lated for each spectrum. The instrumental wavenumber accu- racy was estimated to be 0.01 cm-', by calibration against standard water absorption lines. Using the same experimen- tal apparatus, we have also recorded the polycrystalline spec- trum of 7-A21 as a CsI pellet in the range 100-4OoO cm-'. 7-AZI is a planar asymmetric rotor and the vibrational bands can be classified as A, B, hybrid A/B or C types according to the direction of the vibrational transition moment with respect to the inertial frame. The experimental band-origin wavenumbers are listed in Table 1. The error limits are related only to the uncertainty in the location of the band origins and not to spectral resolution.SQM Vibrational Frequencies and Intensities The HF-SCF equilibrium geometries, the force fields and the dipole moment derivatives of 7-A21 and the parent molecules pyridine and pyrrole, have all been calculated using a stan- dard set of 6-31G**13 orbitals and the suite of computer pro- grams CADPAC, version 514 for quantum chemistry. We followed the procedure described in detail in ref. 9. The set of internal coordinates listed in Table 2, with the atoms labelled as in Fig. 1, was chosen for 7-AZI while the internal coordinates of the two parent molecules, pyridine and pyrrole, were taken from ref. 15 and 16. The scaling factors for the force fields of the parent mol- ecules were determined at the theoretical equilibrium geome- tries, by a Newton-Marquard non-linear fitting procedure : these are listed in Table 3.Compared with those of Xie et our scaling factors for pyrrole differ only marginally 12 I 13 15 Fig. 1 Numbering of atoms in 7-azaindole J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Experimental and computed vibrational spectrum of 7-azaindole vapour IR" calculated transition moment/a.u.' freq. band freq. int. mode /cm -int. shape /cm-' A B C /kmmol-' sym. PED' 216.5(5) m C 221.5 -0.655 18.1 A 63S3, + 22S3,'39 204(30)d 233.5 0.047 0.09 A" 61S3, + 19s38 4-18S3,'38 420.3(3) m B 419.2 -0.242 -0.428 10.2 A' as31 + 13s36 + 1os7'27 456.08(6) s C 461.5 -1.244 65.4 A" 62Sz, + 22s33 + 2os3,'3 7 468.8(1) s C 411.4 -0.984 40.9 A as,, + 36Sz8'36 552.ql)' 549.0 0.001 0.096 0.4 A 68S30'26 580.6(1) w C 567.7 -0.318 4.2 A" 37s34 + 37S32 + 24338'3 5 60511) vw ? 612.5 -0.042 0.1 A" 49s37 + 45s38'34 623.6(2) w A + B 622.1 -0.293 -0.165 4.8 A' 34S31 + 31s36 + 14S35 + 1osz'25 718.1(1) vs C 718.3 1.373 79.7 A" 59sz6 + 27Sz7'33 758.3( 1)' 755.2 0.211 -0.394 8.4 A' 17s30+ 15S, 4-14sz + 4-11s6+ llsl'24 774.29(5) s C 771.7 1.052 46.8 A" 52s2, + 15s~~'32 798.49(5) m C 803.0 -0.359 5.4 A" 49s3z -k 19s37'3 1 869.6(1) w B 888.0 -0.098 -0.229 2.6 A' 45szg + 15sg + 13s31'23 898.24(3) m A 896.1 0.615 0.107 16.5 A' 35s35 + 25s36 + 21s1'2 2 925.q1) m C 873.7 0.108 0.5 A" 72Sz7 + 39sz6'30 944.1(1) m C 948.2 -0.113 0.5 A 59S,, + 46Sz,'29 96035) w C 967.2 -0.115 0.5 A 45S2, + 40Sz, + 34Sz, -12S3,'28 1032/ 1039.0 0.025 0.183 1.4 A 49S, + 18S, + lOS,,'2 1 1063.9(2) w A + B 1058.9 -0.390 0.713 27.9 A' 5os10 + 24s~~+ 11sz0'20 1083.3(2) w A + B 1085.4 -0.032 0.566 13.6 A' 34Sz1 + BSzo + 15s8'19 1115.5(1) w A + B 1108.1 -0.440 -0.120 8.8 A' 28Sl8 + 18S, + 18S,,'18 1200.6(1) w A + B 1192.6 -0.336 -0.146 5.7 A' 2os7 + 19sz9 + 12s1, + 11sz0'17 1252# 1238.6 -0.097 -0.069 0.6 A' 31s3 + 18S17+ 12sz,'16 1284.3(1) s A 1287.5 -0.375 0.087 6.3 A' 25s19+ 16Sg '15 1308.5(5) m B 1269.9 0.451 0.910 43.6 A' 3osz 4-17s3-k loszz'14 1350.3(1) m ? 1358.1 -1.127 0.159 54.7 A 21S17+ 18S1 + 13Sz1'13 1413.3(1) s A + B 1423.5 -0.718 -0.347 26.9 A 22Szz + 21S17+ 20S,, + 14.9,'12 1425.3(1) s A + B 1440.8 -1.196 0.141 61.3 A' 18S,8 + 16Slg + 16S,'11 1495.q2) w A + B 1507.5 0.495 -0.534 22.4 A' 18S5+ 17s18 + 13Sg + 12S17'10 1509.5(5) m B 1528.2 0.092 -0.820 28.8 A' 42s8 + 13Szl + IIS35 + 10s~'9 1579.7(2) m B 1594.4 0.088 0.988 41.6 A 20s1 + us, + 10s6'8 1607.4(2) m A + B 1608.5 -1.073 0.044 48.8 A' 23s6 + 16Ss + 13S19 + 11Szv7 3042(1) w -3054.7 0.471 0.331 14.0 A' 77Sl1 -k l6Sl2'6 3066(2) m -3064.5 0.345 0.663 23.6 A' 79s13 + 14s11'5 3085(2) w -3088.0 0.751 -0.260 26.7 A 78S1, + 13S13'4 3100(2) w -3133.7 -0.138 -0.150 1.8 A 66S14 + 34S15'3 3142.1(3) w A + B 3153.6 0.409 0.137 7.9 A 66S15 + 33S1,'2 3517.5(5) s B 3531.3 0.677 -1.552 121.2 A 1ooS1,'1 " Numbers in parenthesis are uncertanties in units of the last digit.Components of the dipole transition moments in the molecular inertial frame.Only PED contributions 2 10% are quoted. Estimated from microwave spectra.' 'Experimental value from UV absorption spec-tr~m.~f Experimental value from SVLF spectrum.' # Experimental value from polycrystalline IR spectrum of 7-AZI as a CsI pellet. owing to the better quality of our atomic orbital set leading deformation of the pyridine ring. The frequencies evaluated to small modifications in the computed equilibrium from the scaled force field are listed in Table 1. The compari- geometry. With the optimized scaling factors, all experimen- son with the experimental spectrum is satisfactory and is dis- tal fundamental frequencies of pyridine and [2H5]pyridine,17 cussed in detail in the next sections.of pyrrole and [2H,]pyrrole'8 are reproduced with a root- mean-square deviation (RMSD) of 11.5 and 11.9 cm-', Infrared Spectrarespectively. After this work was completed, new IR data became avail- The gas-phase and solid-state spectra of 7-AZI from 450 to able for pyrrole" leading to better estimates of the band 3550 cm-' are compared in Fig. 2. Their intensities differ origins for all fundamentals. The new frequencies differ at mainly in the region from 550 to 650 cm-' and in the region most by 20 cm-' from the values used for the refinement of above 3000 cm-'.The rotational band contours are better the force field and confirm all previous assignmentsI8 of the shown in Fig. 3, where the gas-phase spectrum is plotted fundamentals.These frequencies modify our scaling factors using an expanded scale. The gas-phase and the solid-state only slightly with negligible effects on the calculated IR spec-spectra from 180 to 450cm-'are shown in Fig. 4. trum, so their inclusion was not felt important for the The rotational contours of most bands are easily recog- analysis of the IR spectrum of 7-AZI. nized, apart from a few cases that show strong overlapping or The scaling factors of the parent molecules were next trans- very low band intensity. Although the spectra have been mea- ferred to the corresponding internal coordinates of 7-AZI as sured in vacuum, several very sharp lines are observed in the listed in Table 2. The scaling factor for the C1-C6 stretching gas-phase spectra from 1300 to 2000 cm-' and from 180 to coordinate (see Fig.l), was taken to be equal to that of the 400 cm-', due to the absorption of the residual H20 in the C-C pyridine stretches and that for the butterfly motion of cell and in the interferometer. These are easily distinguished the two rings was assumed to be equal to that of the A, from the 7-AZI bands. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Internal coordinate definition for 7-azaindole and scaling factors applied in the prediction of the vibrational spectrum definition description scale factor 1 C-C stretch for bond common 0.8434 2 3 to both rings C-N stretches (pyridine ring) 0.7523 4 5 C-C stretches (pyridine ring) 0.8434 6 7 8 C-C stretches (pyrrole ring) 0.7825 9 10 C-N stretches (pyrrole ring) 0.8502 11 12 C-H stretches (pyridine ring) 0.8357 13 14 C-H stretches (pyrrole ring) 0.8428 15 16 N-H stretch 0.8053 17 18 C-H rocking (pyridine ring) 0.8310 19 20 C-H rocking (pyrrole ring) 0.8323 21 22 N-H rocking 0.7593 23 C-H wagging (pyridine ring) 0.7709 24 25 26 C-H wagging (pyrrole ring) 0.7521 27 28 N-H wagging 0.9098 29 trigonal deformation 0.8458 (pyridine ring) 30 asymmetric deformation 1 0.8289 (pyridine ring) 31 asymmetric deformation 2 32 33 34 35 t(3, 2, 1, 6) -t(2, 1, 6, 5) + t(1, 6, 5, 4) ~(3,2, 1, 6) -~(1,6, 5, 4) -t(6, 5, 4, 3) + t(5,4, 3, 2) -t(4, 3, 2, 1) +~(6,5, 4, 3) -~(4,3, 2, 1) +2t(5, 4, 3, 2) -t(6, 5, 4, 3) -~(4, 3, 2, 1) 2t(2, 1, 6, 5)-t(3, 2, 1, 6) -t(1, 6, 5, 4) 441, 9-81 + ~0~(1440)~449,8,7)+ 446, 1,911 +cos(72")[&8, 7, 6) + 447, 6, l)] puckering (pyridine ring) (pyridine ring) asymmetric torsion 1 (pyridine ring) asymmetric torsion 2 (pyridine ring) ring deformation 1 (pyrrole ring) 0.7872 0.7345 0.8458 36 37 38 +[I -~0s(144")1[&8, 7, 6) -d47, 6, I)] +cos(144")[z(9, 8, 7, 6) + t(7, 6, 1, 9)] +cos(72")[~(1, 9, 8, 7) + 46, 1, 9, 8)] [COS(144") -~0S(72")][&9, 8, 7) -&6, 1, 9)] t(8, 7, 6, 1) [~0s(144")-~0S(72")1[~(7,6, 1, 9) -t(9, 8, 7, 6)] ring deformation 2 (pyrrole ring) ring torsion 1 (pyrrole ring) ring torsion 2 (pyrrole ring) 0.8264 39 +[1 -~0S(144")][~(6,1, 9, 8) -~(1,9, 8, 7)] 45, 6, 1, 9) -47, 6, 1, 2) butterfly mode ortho-coupling (pyridine ring) meta-coupling (pyridine ring) para-coupling (pyridine ring) 0.8458 0.7642 0.5233 0.5002 ~~~~ ~ ~ n Here r(a,b) is the bond length between atoms a, b; &a, b, c) is the angle abc; a(a, 6, c, d) is the angle between the line passing through atoms a, b and the plane defined by b, c, d; r(a, b, c, 6)is the dihedral angle between planes defined by a, b, c and b, c, d, respectively.Many sequences of the transitions arising from the large amplitude motions are apparent in the strong bands at 216, 456.08,468.8, 718.29, 774.29 and 798.49 cm- '. Vibrational Assignments and Analysis The final comparison of all computed and experimental fre- quencies is shown in Table 1. The RMSD of the 39 funda- mentals is 18 cm-'.We included in the comparison five experimental frequencies obtained from the analysis of microwave' and electroni~~*~ spectra. The global RMSD reduces to 8 cm- if we exclude five fundamentals: v3, v14, vJO, vf6 and v38 whose deviation from the calculated wave- numbers is greater than 20 cm-'.From low to high wave- numbers the following comments are in order. Spectral Region around 220 cm-' Only one band with a C-type rotational contour is observed at 216. In the solid-state spectrum the band of medium inten- sity at 236 cm-' has a shoulder at 232 cm-'.From the inten- sity of the vibrational satellites in the microwave spectrum' and from the shifts of the second moment of inertia, the origin of the butterfly motion has been estimated at 193 k30 cm-' and the out-of-plane bending along the A axis at 204 30 cm-'.In the UV spectrum5 two long sequences, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Optimized scaling factors for pyridine and pyrrole pyridine ~~ pyrrole internal coordinate factor internal coordinate factor stretching C-H stretching C-C stretching C-N rocking C-H wagging C-H A, ring deformation B, ring deformation B, ring torsion A, ring torsion ortho-coupling meta-coupling para-coupling 0.8357 0.8434 0.7523 0.83 10 0.7709 0.8458 0.8289 0.7872 0.7346 0.7642 0.5232 0.5002 stretching N-H stretching C-H stretching C-C stretching C-N rocking N-H rocking C-H wagging N-H wagging C-H ring deformations ring torsions 0.8053 0.8428 0.7825 0.8502 0.7593 0.8323 0.9098 0.7521 0.8458 0.8264 generated by ground-state vibrations of about 200 cm-', have been observed.In conclusion, all evidence points to the presence, in this region, of two fundamentals of A" symmetry. With our scaled force field we predict in this region the out-of-plane butterfly motion at 221.5 cm-' and an asym- metric torsion of the six- and the five-membered rings at 233.5 cm-', with the latter vibration carrying zero intensity. Therefore, based on computed frequencies and intensities we assign the band at 216.5 cm- 'in the vapour spectrum to which corresponds to the butterfly motion. The experimental wavenumber for v38 comes from the microwave spectrum and our theoretical value is within the experimental uncer- tainty.Spectral Region around 450 cm-Here the gas-phase IR spectrum is not easy to interpret. One B-type and three C-type bands are observed at 420.3, 456.08, 468.8, and 475.2 cm- ',respectively. The solid-state spectrum also presents four bands, while the analysis of the single vibrational level fluorescence (SVLF) spectra' places an A' 16 14 12 10 8 6 4 722c. .-co =I 3550.0 2775.0 fundamental at 432 cm-'. The latter value differs by more than 10 cm-' from the wavenumber of the corresponding IR band. With SQM we predict three fundamentals in this region: two strong of A" symmetry, at 411.4 and 461.5 cm-', and an A' of medium intensity at 419.2 cm-'. The assign- ments reported in Table 1 are based on the comparison between computed and experimental intensities and band contours.Spectral Region around 600cm-' We predict four weak fundamentals in this region: three of them are assigned to weak features in the gas spectrum (see Fig. 3). The only plausible assignment for the A' fundamental corresponding to an asymmetric deformation of the six-membered ring and calculated at 549.0 cm-' with very low intensity, is the hot band at 552.0 cm-' observed in the UV absorption ~pectrum.~ Spectral Region 700-970 cm' ' The most intense fundamentals are observed in this part of the spectrum. The strong C-type bands at 718.1 and 774.29 cm-' are assigned to the C-H wagging vibrations estimated at 718.3 and 771.7 cm-', respectively. Three additional C-H wagging vibrations of A" symmetry are calculated at 873.7, 948.2 and 967.2 cm-' with weak intensity.We favour their assignments to the weak and the medium C-type bands at 925.0, 944.1 and 960.5 cm-', respectively. Owing to the large difference between the computed and the experimental fre- quency of the first band (ca. 50 cm-') the alternative assign- ment of this fundamental to the very weak band at 853.9 cm-' has been considered: this would give better agreement between computed and experimental frequencies but would leave unassigned the 925.0 cm- 'band, for which no plausible frequency combinations have been found. Moreover, the presence in the spectral region 1610-2000 cm-' of com-bination bands involving the 925.0 cm- frequency supports the assignment of this band as a fundamental.With this 2000.0 1225.0 450.0 3550.0 2775.0 2000.0 1225.0 450.0 wavenumber/cm-' Fig. 2 Comparison of two IR spectra of 7-azaindole from 450 to 3550 cm-': (a) polycrystalline phase, resolution 0.5 cm-';(b) vapour phase, resolution 0.18 an - J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3550.0 3356.3 3162.5 2968.8 2775.0 ~ ~ ~ ~~~~~~~ nv) C.-t 8-g 6-Y s 4-c..-v) C5 2-.-0 1 1 I 2000.0 1806.3 1612.5 1418.8 1225.0 Oi I I I I I 1225.0 1031.3 837.5 643.8 450.0 wavenumber/cm-Fig. 3 Expanded plot of the vapour-phase IR spectrum of 7-azaindole from 450 to 3550 cm-'(resolution 0.18 cm-') 5f I I I I 450.0 382.5 315.0 247.5 180.0 wavenurnber/cm -' Fig.4 Comparison of the two IR spectra of 7-azaindole from 180 to 450 cm-':(a) polycrystalline phase, resolution 0.5 cm-';(b) vapour phase, resolution 0.20cm-' 3218 assignment, the band at 853.9 cm-' is then assigned to the combination ~14-v37 (1308.5 -456.08 = 852.4 cm- '). The C-type band of medium intensity at 798.49 cm-' is assigned to the six-membered ring-puckering-motion coordinate, v3 ', which is calculated at 803.0 cm- '. Three additional A' funda-mentals, mainly deformation modes of both rings, are calcu- lated at 755.2, 888.0 and 896.1 cm-'. Only the highest frequency mode is clearly identified in the IR spectrum as due to the pure A-type profile of the band at 898.24 cm-', in agreement with the computed transition moment.The weak B-type band at 869.6 cm-' corresponds to the fundamental calculated at 888.0 cm -'. The low-frequency band, probably hidden under the intense C-type bands in this region, is not observed in the IR spectrum: the hot band at 758.5 cm-' in the UV absorption spectrum' is a likely candidate for this assignment. Spectral Region 1040-1210 cm" Five weak A' fundamentals are predicted in this interval, with mainly C-C stretching and C-H rocking character. Four of these fundamentals are easily identified from their intensities and rotational profiles. The missing fundamental, calculated at 1039.0 cm- ' with a very weak intensity, is identified with the band at 1032 cm- ' in the SVLF spectrum.' Spectral Region 1250-1610 cm" As in the preceding region, only totally symmetric fundamen- tals are observed here: three out of nine have strong, and the others medium, intensities, with main contributions from the C-C stretching, C-H rocking, C-N stretching and N-H rocking coordinates.In the theoretical spectrum five out of ten bands are computed to be strong and the others are of medium-strong intensity. Only for the weakest of these fun- damentals, calculated at 1238.6 cm- ',were we unable to find a correspondence in the IR spectrum but the spectrum of the solid reveals a plausible candidate for this fundamental at 1252 cm-'. For the assignment of the bands at 1284.3 and 1308.5 cm-'as v15 and ~14we note that only the rotational contours of these bands agree well with our prediction and that the total theoretical intensities and the frequency of v14 show larger deviations (38 cm-') from the experimental values.We are aware that, for a molecule of this complexity, an assignment based only on the observed contour could be incorrect since the rotational contour may be distorted by perturbations,20 and therefore we cannot entirely exclude the inverse ordering of the two bands. Spectral Region 1610-2000 cm-' In this part of the spectrum many combination and overtone bands are observed with weak intensity. In Table 4 we list the experimental wavenumbers with the proposed assignments. Spectral Region 3040-3520 cm-* Six fundamentals are observed in this region: five C-H stretching, three of which are from the six-membered ring and two from the five-membered ring and the N-H stretch-ing.The three fundamentals of the pyridine ring form a com- posite band, where discontinuities in the profile can be identified as Q-branches of A-type bands. Both the C-H stretching bands of the five-membered ring are weak and only that on the high-frequency side, observed at 3142 cm-' and calculated at 3154.7 cm-', is easily identified and as- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Experimental wavenumbers and assignments in the spec- tral region 1610-2000 an-' ~~~~~~~~ ~~~ frequency band /cm-' intensity shape assignment 1623-8(4) vw A + B ~30 (925.0 + 718.1 = 1643.1)+ ~33 1652.2(5) w A + B vZ9 + v~~ (944.1 + 718.1 = 1662.2) 1705.1(5) w ? vZ9 + v~~ (944.1 + 774.29 = 1718.1) 172q 1) vw ? ~28 (960.5 + 774.29 = 1734.8)+ ~32 1856.9(5) w A + B 2v3, (2 x 925.0 = 1850.0) v~~ + vZ9 (925.0 + 944.1 = 1879.1) 1886.5(5) w B 2v2, (2 x 944.1 = 1888.2) 19 19( 1) w A + B 2vZ8 (2 x 960.5 = 1921.0) signed as v2.The experimental wavenumber of the remaining C-H stretching fundamental, v3, is apparently observed as an inflexion at 3100 cm-', with 30 cm-' difference from the calculated value. Owing to the low intensity computed for this band we take this assignment as tentative, as all the other five X-H stretching fundamentals appear to be very well predicted by theory. The N-H stretching is one of the strongest fundamentals: it is of B type, observed at 3517 cm-' and calculated at 3531 cm-'.Conclusions The gas-phase IR spectrum of 7-azaindole has been recorded from lo0 to 4000cm-'. Spectral assignments are based on the SQM method: using a convenient set of internal coordi- nates, the scaling factors for the theoretical force field were determined by least-squares fitting of the theoretical to the experimental frequencies of the parent molecules pyridine and pyrrole and their perdeuteriated isotopomers. The final RMSD of the observed and calculated fundamen- tals is 18 cm-', better than that obtained for indazole' using the same approach. In particular the deviations of the C-H and N-H stretching frequencies are 10 and 14 cm-', respec-tively, while for indazole they were 30 and 50 cm-'; the dis- crepancies for the bands v36 and v37 corresponding to the N-H wagging motion are 60 and 5 cm-', while for indazole the deviation was greater than 100 cm-'.If we exclude from the comparison the vj , v14, ~30,v36 and v38 fundamentals, whose frequencies differ from the calculated values by more than 20 cm-', the RMSD reduces to 8 cm-'. These results confirm the validity of the SQM method for spectral IR assignments of complex molecules. Financial support from CNR within 'Progetto Nazionale di Informatica Chimica' and 'Progetto Calcolo Avanzato in Chimica', MURST, and from the EEC under the 'Human Capital Mobility Program' (contract N. ERBCHRXCT93- 01 57) is gratefully acknowledged. References W. Caminati, S. Di Bernard0 and A. Trombetti, J.Mol. Struct., 1990,223,415. K. Fuke, H. Yoshiuchi and K. Kaya, J. Phys. 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