CI Nuclear Quadrupole Coupling Tensor in Phosgene (6) R. J. Holman and M. J. Perkins, J . Chem. Soc. 6 ,2195 (1970). (7) J. G. Aston, D. F. Mevard, and M. G. Haryberry, J. Chem. Soc., 1530 (1932). (8) R. N. Haszeldine and B. R. Steel, J . Chem. Soc., 1199 (1953). (9) R. N. Haszeidine, J . Chem. Soc., 4423 (1952). (10) A. Mackor, Th. A. J. W. Wajor, and Th. J. de Boer, Tetrahedron Lett., 19, 2115 (1966). (1 1) Th. A. J. W. Wajor, A. Mackor, Th. J. de Boer, and J. D. W. Van Voorst, Tetrahedron, 23, 4021 (1962); S. Forshult, C. Lagercrantz, and K. Torsell, Acta Chem. Scand., 23, 522 (1989). (12) J. W. Hartgerink, J. 6. F. N. Engberts, and Th. J. de Boer, Tetrahedron Letf., 29, 2709 (1971). (13) K. Morokuma, J . Am. Chem. Soc., 91, 5143 (1969). (14) G. R. Underwood and V. L. Vogel, Mol. Phys., 621 (1970).
The Journal of Physical Chemistty, Vol. 83, No. 24, 1979 3 1 6 1 (15) G. R. Underwood and V. L. Vogel, J . Am. Chern. Soc., 92, 5019 (1970). (16) I. Morishima, K. Endo, and T. Yonezawa, Chem. Phys. Lett., 9, 143 (1971). (17) The coupling constants of (CF,),Nt) and (CH,),NO experimentally are reported by W. R. Knolle and J. R. Bolton, J. Am. Chem. SOC.,91, 5411 (1969), and I. Morishima et al., J . Phys. Lett., 9, 143 (1971), respectively. The coupling constants of all spin adducts were calculated by the INDO method In the present work. (18) C. Heller and H. M. McConnell, J . Chem. Phys., 32, 1535 (1960). (19) P. J. Krusic, P. Meakin, and J. P. Jessen, J . Phys. Chem., 75, 3438 (1971). (20) J. C. Scaiano and K. U. Ingold, J. Phys. Chem., 80, 275 (1976). (21) J. K. Kochi, Adv. Free Radical Chem., 5, 189 (1975).
Determination of the Chlorine Nuclear Quadrupole Coupling Tensor in Phosgene by Microwave Spectroscopy Arthur C. Ferguson and W. H. Flygare" Department of Chemistry, University of Illinois, Urbana, I/iinois 6 180 1, and Department of Chemistry, Worcester State College, Worcester, Massachusetts 0 1602 (Received December 18, 1978)
The complete chlorine nuclear quadrupole coupling tensor has been measured in phosgene by using high-resolution microwave spectroscopy. The principal axis system makes an angle of 57.6 f 0.3' from the b inertial axis compared to the angle of 55.65 f 0.5O made between the C-C1 internuclear line and the same b inertial axis. A discussion of the chemical bonding is included.
Introduction The presence in phosgene of chlorine atoms bonded to a carbon atom which is also engaged in a double bond results in a situation in which H bonding between the chlorine atoms and the carbon atom is possible as illustrated by resonance structures of the form 0 I
C I $
c1
C1'
The extent of this H bonding can be estimated, using a method based on the theory of Townes and Daley1S2(see ref 3, section 14,11c),from the values of the diagonal elements of the quadrupole coupling tensor x a t a chlorine nucleus defined in the C-C1 bond axis system. In his original investigation of the microwave spectrum of phosgene Robinson4 found that the determination of the complete x tensor a t a chlorine nucleus, which is necessary for the determination of the elements of that tensor in the C-Cl bond axis system without the additional assumption of coincidence between the C-C1 bond axis and one of the principal axes of the x tensor, required a degree of resolution and precision of line measurement which was beyond the capabilities of the spectrometers available to him at the time. The objective of the present work was to determine the complete x tensors a t the chlorine nuclei is phosgene by taking advantage of the very high resolution and precision obtainable with the spectrometer available in this laboratory and to interpret the elements of these tensors in terms of the bonding between the carbon and chlorine atoms. Address correspondence to this author at the University of Illinois. 0022-3654/79/2083-3161$01.OO/O
Experimental Section The spectral line measurements on both C035C12and C035C137C1 were made by using a sample of phosgene containing the isotopes of chlorine in natural abundance which was obtained from the Matheson Co. and which was used without further purification. Except for a moderately intense peak a t approximately 2080 cm-I which suggested the presence of carbon monoxide, the infrared spectrum of this sample showed no sign of impurities. The microwave spectrometer used in this investigation, employing a phase-stabilized klyston signal source and a 12-ft S band waveguide cell, has been described e l ~ e w h e r e .The ~ # ~only modification made in this apparatus for the present work arose from the low frequencies of the lines measured and consisted of locking the klystron directly to a harmonic of the VFO rather than to the sum of a harmonic of a lower frequency klystron and the fundamental of the VFO. Each of the line frequency values found in Table I is based on several sweeps made both in the direction of increasing and the direction of decreasing frequency and averaged so as to ensure that the final values were free from instrumental effects. The experimental error quoted in Table I represent 95% confidence limits calculated from the standard deviations of the up frequency and down frequency sweeps. Analysis of the Spectrum Introduction. The interpretation of the quadrupole hyperfine structure in the microwave spectrum of a molecule containing two quadrupolar nuclei can be made in terms of the quadrupole coupling tensors x(1) and x(2), xcl = CQ ((a2V/ax,axj),of the two quadrupolar nuclei defined in the inertial principal axis system of the molecule. The methods used for this interpretation are described in some detail by Krusic7 and Flygare and Gwinn.8 Since the two chlorine nuclei in phosgene both have spin 312 and 0 1979 American Chemical Society
3162
The Journal of Physical Chemistty, Vol. 83,No. 24, 1979
TABLE I: Observed Transitions in C035Cl,and C035C137C1 est uncertainty AVobsd ; Vobsd> in Vobsdy Avcalcd, J+ J' I,F+ I',F' MHz MHz MHz ~~
~
~
co c1 35
1,"
1,l
1,l
3,4 3,2 3,3 1,l 3,4 3,3 1,2
3,4 3,3 3,2 1,0 3,3 3,4 1,2
6727.720
io.010 20.003 t0.004 i0.005 t0.006 k0.003 t0.003 k0.004 i0.007
0,2
0,2 2,2
6730.504 6735.359
20.005 * 0,009
b 0.003
212
2.2
212
{$$}
6745.715 6748.438
i0.006 i0.006
0.000 -0.018
l,,
3,4 2,3 3,3 3.3
3,4 2,3 3,2 3.4
5500.562 5506.281 5507.394 5514.008
20.003 i0.003 i-0,006 i0.004
-0.023' b -0.068' -0.003
2,,
1,2
{i::} 6669.131 3.2
to.010
- 0.001
6672.766 6675.696 6676.738 6677.456 6679.259 6682.475
20.014 i- 0.008 i0.005 t0.008 i0.005 i0.007
-0,008 0.009 0.003 -0.002 b 0.005
6688.532
io.009
0.012
6689.143
i0.006
-0.011
2,,
0,2
{;;;I
5493.637 5500.076 5501.035 5507.885 5509.969 5510.986 5515.165 5517.309
b - 0.008
- 0.005 -0.004 0.020 0,009 0.007 - 0.003
-0.012
co35c137c1
0,2 3,5 3,2 1,3 2,4 3.2 2:2
0:2 3,5 3,3 1,3 2,4 3.1
{ir!} 1-
3,3
3,2
Transitions a A v is the quadrupole hyperfine splitting. from which the splittings were measured. ' These transitions were sufficiently overlapped by stronger transitions belonging to C03'C12 that their apparent peak positions may deviate significantly from the true peak positions. For this reason these transitions were not used in the calculation of the x tensor elements.
experience identical (C035C12) or nearly identical (C035C137C1)coupling, the most convenient representation in which to calculate the matrix elements required for this interpretation is the IIIzJFM r e p r e ~ e n t a t i o n . ~The t ~ expressions for these matrix elements which were used in the present work are those given by Krusica7 Since the phosgene molecule has a plane of symmetry which contains the a and b inertial axes, the quadrupole coupling tensors of the chlorine nuclei have only one nonzero off-diagonal element, Xab. Moreover, since the x tensors are traceless, only two of the three diagonal elements of each tensor are independent. For the present work it was decided to treat xu, and xccas the two independent diagonal elements. The first-order quadrupole shifts and splittings are completely determined by the diagonal elements xu, and xCc.' Therefore, only those diagonal elements can be determined directly from an analysis of the first-order effects. Xab can be determined either from direct measurement of Xab-dependent second-order effects or by first determining the diagonal elements of the x tensors in C035C12and C035C137C1 from first-order effects and then calculating the values of Xab from those diagonal elements and the 0.897' angle (calculated from data in ref 4) between the inertial principal axis systems in C035C12and C035C137C1.In the
Ferguson and Flygare
absence of sufficiently large xnb-dependentsecond-order effects to allow a direct determination of Xnb the latter approach was followed by using a method similar to that described in ref 10. The rotational transitions examined were lol lloand 202 211, which were chosen because their unusually large quadrupole splittings facilitated a precise determination of the diagonal elements of the x tensor. Procedure. In outline, the procedure used to obtain the x tensor elements from the experimentally measured quadrupole splittings was as follows. First, the second-order contributions to the splittings were computed by using preliminary values of the x tensor elements obtained from Robinson's work4 and the assumed coincidence between the a axis of the x tensor and the C-C1 internuclear axis. These second-order contributions, along with the effects on the peak positions caused by incompletely resolved lines belonging to the same isotropic species,ll were subtracted from the measured splittings, and the corrected splittings were then used to obtain new values for the diagonal x tensor elements by means of a least-squares fitting procedure. From these new values for the diagonal elements and the angle between the inertial principal axis systems in C035C1, and C035C137C1new values of Xab were calculated. The cycle of calculations was then started again with the recomputation of the second-order contributions to the splittings by using the new set of values of the x tensor elements and was repeated until the variation in the values of these elements from one cycle to the next became negligibly small. Since the two chlorine nuclei in C035C12experience identical quadrupole coupling, only two parameters, xu, and xcc,are needed to describe the first-order contributions to the quadrupole splittings for C035C12.Moreover, since these first-order contributions depend on xaa and xccin a linear fashion, linear least-squares techniques could be used to obtain values of xu, and xccfrom the corrected first-order splittings without complication. In C035C137C1 35x,a,35xcc, 3 7 ~ , u , and 37xcc are all different, and the first-order splittings are linear in none of them. The number of variables which had to be dealt with in analyzing these first-order splittings was reduced from the apparent four to two by making use of the fact that, since the electronic environment of the chlorine nuclei should be the same in C035C12and C035C1zC1(for a demonstration of this in the closely related molecule thiophosgene see ref 25) and since the rotation of the inertial principal axes between C035C12and C035C137C1occurs in the ab plane, one may assume that 35xcc(C035C137C1) = xcc(C035C12)and that 37xcc(C035C137C1) = xcc(C035C12) / 1.26878, where 1.26878 is the ratio 35Q/37Q as given by Livingston.12 Therefore, in each cycle of calculation the first-order contributions to the quadrupole splittings for C035C137C1 were fit holding 35xcc and 37xccconstant at the values obtained from the most recently calculated value of xccfor C035C12and varying only 3 5 ~ , n and 3 7 ~ , a . Since the first-order contributions to the splittings are not linear in 3 5 ~ , a and 37xuu, the procedure for fitting those contributions consisted of computing values for them on the basis of the most recently obtained values of 35xuu, 35xcc,3 7 ~ n u , and 37xccand assuming that the differences between the computed and experimentally determined first-order contributions could be linearly related to corrections in 35xu, and 3 7 ~ u aby the equation
-
-
CI Nuclear Quadrupole Coupling Tensor in Phosgene
The Journal of Physical Chemistry, Vol. 83, No. 24, 7979 3163
TABLE 11: Values of the x Tensor Elements in the Inertial Principal Axis System
__-
co35c137c1 c035c1,
MHz x b b , MHz x c c , MHz X a b , MHz Xaa,
-37.641 i 0.025” 10.337 t 0.029 27.304 t 0.016’ 51.0 i 1 . 6
3 5 c 1
3 7 c 1
- 36.009 i
0.036’ 8.705 * 0.039 27.304 i 0.016“ -51.8 i 1 . 6
-30.899 1 0.067“ 9.379 t 0.069‘ 21.520 i 0.013 39.6 i 1 . 2
” These estimated uncertainties are twice the standard deviations in the associated quantities as obtained from the leastsquares fitting program. All of the other estimated uncertainties are derived from these by the method for the determina. tion of the propagation of probable error. The values of AveXpt - b&V,,]cd were then fit to values of A35xaaand A 3 7 ~ , aby linear least-squares techniques. Because of the approximate nature of eq l, the steps involved in fitting the first-order contributions, to the splittings in C035C137C1were repeated several times as a subcycle within each main cycle of the calculations. Once values for the diagonal x tensor elements for both chlorine nuclei in C035C137C1have been obtained, it is possible to make two independent calculations of X a b in C035Cl,, one by using the elements for the 35Clnucleus in C035C137C1,the other by using the elements for the 37Cl nucleus. The value of X a b for C035C12given in Table I1 is the average of the results of these two calculations. Table I1 gives the values obtained for the x tensor elements for C035C12and C035C137C1. Computer Programs Used. The program used for computing the first-order contributions to transition frequencies and the relative intensities of those transitions used in the initial assignment of the spectra and in the fitting of the spectrum of C035C137C1 is very similar to the one written by Krusic7 except that it employs the III,JFMF representation for the matrix elements rather than the IIJFIIIFMFrepresentation used by Krusic. The program for computing to second order the quadrupole contributions to the transition frequencies is similar in format to the first-order program except that the matrix is expanded to include blocks off diagonal in J. The matrix elements of the squares of direction cosines of the form (J,Jlazg21J,J),g = a, b, c, required for the first-order calculationsg were obtained by means of a rigid rotor program. The matrix elements of the squares and products of direction cosines of the form (J,MJ = J < l a ~ ~ c u z ~M, JJJ=’ ~J < ) ,where J , is the smaller of J and J’, required for the second-order calculations were obtained by means of the equation (J,MJ = J
