MILLIMETRE WAVE SPECTRUM OF METHYL MERCURY CHLORIDE * BY J. T. Cox, T. GAUMANN -f AND W. J. ORVILLE THOMAS $ Received 21st January, 1955 Dept. of Physics, Duke University, Durham, N.C., U.S.A. The J = 16 -+ 17 rotational transition has been studied for CH3HgCW for the most abundant mercury isotopes. These measurements lead to new values for the spectral constants Bo and Djk and to values for Djj the distortion constant associated with end- over-end rotation. Previous studies 1. 2. 3 on mercuric compounds have indicated that the a-bonding orbitals of the mercury atom are sp-hybridized. It was shown by Gordy and Sheridan 4 from the pure rotation spectra of methyl mercury chloride and bromide that these molecules are symmetrical tops. This indicated un- equivocally that in them the C-Hg-Halogen grouping is strictly linear.In their study the Bo's were calculated from the general synvnetric top formula neglecting the centrifugal distortion term 4Djj(J + l)3. This term has now been measured for the methyl mercury chloride molecules containing Hg (198, 199, 200 and 202), and new values obtained for the rotational constants. Hg201 is the only Hg isotope with a nuclear quadrupole moment. Because of its small natural abundance (13.2 %) and the splitting of the lines by nuclear quadrupole coupling the intensity of the lines is expected to be small and the assignment of the CH3Hg201C1 spectrum difficult. An attempt has been made to unravel the spectrum and an indication of the probable value of the Hg201 nuclear quadrupole coupling constant obtained. YO 2B(J + 1) - 4Djj(J + 1)' - 2Djk(J + 1)K2, EXPERIMENTAL The sample of methyl mercury chloride was the one used in the previous microwave study by Gordy and Sheridan.4 The microwave spectra were observed with a frequency sweep spectrometer employing a video type detector.5 Crystal harmonic generators 6 driven by reflex Klystron tubes were used as energy sources.Frequencies were measured with a standard monitored by comparison with the standard 5-Mc/s signal of the National Bureau of Standards station WWV. Methyl mercury chloride is a solid with a vapour pressure of 1.6 x 10-2 mm at 27" C.7 The substance was allowed to evaporate into the evacuated wave-guide cell at room temperature. SPECTRAL CONSTANTS The J = 16 --t 17 transition of CH3HgCP was studied.This particular transition was chosen because the splitting by the chlorine coupling is negligible and only one major C1 line is obtained for each K value when AF = 4 1. A large number of frequencies were observed owing to the large number of abundant mercury isotopes and the nuclear "This research was supported by The United States Air Force through the Office f' Present address : Organisch-Chemisches Laboratorium, Eidg. Technischen Hoch- $ Visiting Fulbright Scholar, 1953-54. Present address : The Edward Davies Chemical 52 of Scientific Research of the Air Research and Development Command. schule, Zurich, Switzerland. Laboratories, University College of Wales, Aberystwyth.J . T. cox, T. GAUMANN AND w. J . ORVILLE THOMAS 53 quadrupole hyperfine structure due to Hg201.Several observed lines not accounted for in the course of this work probably arise from molecules in an excited (bending) vibrational state. In tables 1 and 2 are listed the lines observed4 for the J= 8 --t 9 and for the J = 16 -+ 17 rotational transitions of methyl mercury chloride, together with the calculated values obtained using the spectral constants determined from these transitions and given in table 3. TABLE 1 .-FREQUENCIES OF mercury isotope 198 199 200 202 THE LINES OF THE J = 8 3 9 ROTATIONAL TRANSITION OF CH3HgC135 FOR K = 0 vo (obs.) vo (calc.) Mcls Mc/s 37394.00 37393.99 37388.40 37388.40 3738280 37382.80 3737 1 * 60 37371.60 CENTRIFUGAL DISTORTION CONSTANTS Interaction of the rotations of the molecule about the A and B axes (fig.1) is responsible for the energy term containing Djk. Since Djk > 0 the effective moment of inertia Ib of the molecule about the B-axis increases with K. This effect is opposite to that expected B FIG. 1 .--The methyl mercury chloride molecule. on simple grounds, since an increase in K would be expected to decrease a and thus decrease I&. For the methyl halides it has been shown by Thomas, Cox and Gordy 8 that the positive sign of ojk is explicable in terms of a shifting of the internal energy oi the molecule from one bond to another. The mechanism of distortion of the molecule as it rotates is described thus. As K increases so does the angular momentum about the A-axis. The increased centrifugal force acting on the hydrogens of the methyl group tends to make the CH3 group more nearly planar. Effectively the s-character of the three carbon orbitals which bond to hydrogen is increased.To offset this the s character of the carbon orbital bonding to the mercury must be reduced for proper normalization. The combined effect of the hybridization changes at the carbon atom is to increase the bond stretching force constant of the C-H bonds and to decrease that of the G-Hg bond. The effect of changes in size and shape of the CH3 group upon Ib can be neglected in comparison with the effect of changes in the C-Hg bond length. An increase of only 0.0012A in the C-Hg bond length would suffice to account for the 17-89 kc/s separation of the K = 0 and K = 5 lines in the J = 16 -+ 17 transition of CH3HgzooC135. If this increase is due entirely to a change in the covalent radius of the carbon atom it corresponds to a decrease of about 0.7 % in s character (or increase in p character) of the carbon orbital.This small hybrid- ization change would be sufficient to account for the needed C-Hg bond lengthening.01.0 1.12 10.0 =F SPZ.0 10.0 PZ.9LOZ zo$H 01.0 JF 1.12 01.0 ?= 0-12 01.0 0.12 10.0 'f 6SZ.O 10.0 T 9sz.o 10.0 =F TPZ-0 10.0 'f 98.9LOZ 10-0 81-LLOZ 10-0 =F 8P-LLOZ oozZH 661sH 861SHJ. T . cox, T . GAUMANN AND w. J. ORVILLE THOMAS 55 From this change in hybridization an increase of about 22’ in ,/- HCH and a decrease of 0.0004A in the C-H bond length respectively would be expected. The latter would be partly compensated by the centrifugal force tending to lengthen the C-H bonds.Since the molecule is a symmetric top, in the ground vibrational state, the CHgCl grouping is linear and no hybridization changes occur at the Hg atom. This implies that the HgCl bond length does not vary with K and hence the effects decribed are confined to the CH3-Hg grouping. Similar arguments applied to the end-over-end rotation lead one to conclude that the C-Hg would be shortened and DJ negative. Since Djj is positive in value it seems clear that the much more rapid rotation of the molecule about the A axis is more effective in distorting the HCH angles and thus varying the state of hybridization of the carbon atom than is the end-over-end rotation. The effect of the end-over-end rotation would be to decrease the magnitude of Djj. It should be pointed out that the effect of the HGH-angle distortion on the C-Hg bond-length will also tend to be cancelled by the lengthen- ing of the C-Hg-Cl grouping due to centrifugal distortion.This picture of a change in the hybridization ratios of the carbon orbitals as the molecule spins faster around the A axis receives some support from the ideas of Linnett and Wheatley 9910 who have indi- cated the possibility of the bonding orbitals of a central atom changing their hybridization in such a manner as to follow the outer atoms during FIG. 2.-Bending vibration of CH4 molecule dur- ing which orbital following by change of carbon hybridization is possible. bending vibrations. Linnett and Wheatley concluded that bending vibrations will occur more easily if the bonding orbitals are able to follow the movement of the atoms by a change of hybridization.They showed that in methane it is possible for the bonding orbitals to follow the atoms during certain vibrations by change of hybridization. In particular, it was found that for the bending vibration illustrated in fig. 2 the carbon bonding orbitals were able, by change of hybridization, to follow the attached hydrogen atoms. When we examine the motion of the methyl mercury chloride molecule as Kincreases it is seen that the hydrogen atoms move in precisely the same direction with respect to each other as they do in the vibrational bending mode illustrated in fig. 2. This problem was suggested by Prof. Walter Gordy to whom we are very grateful for much useful discussion and encouragement. Dr. Albert Jache is thanked for his technical assistance. One of us (T. G.) thanks the Sweizerischen Nationalfonds und der Gesellschaft fur Stipendium auf dem Gebiete der Chemie for assistance during this work. 1 Braekken and Scholten, 2. Krist., 1934, 89, 448 ; 1932, 81, 152. 2 Braune and Linke, 2. physik. Chem. B, 1935,31, 12. 3 Gutowsky, J. Chem. Physics, 1949, 17, 128. 4 Gordy and Sheridan, J. Chem. Physics, 1954,22,92. 5 Gordy, Smith and Trambarulo, Microwave Spectroscopy (John Wiley & Sons, 6 King and Gordy, Physic. Rev., 1953,90,319 ; 1954,93,407. 7 Charnley and Skinner, J. Chem. SOC., 1951, 1921. SThomas, Cox and Gordy, J. Chern. Physics, 1954,22, 1718. 9 Linnett and Wheatley, Trans. Furaday SOC., 1949, 45, 33. 10 Linnett and Wheatley, Trans. Furuduy SOC., 1949, 45, 39. Inc., New York, 1953), p. 6.