J. Phys. Chem. 1995, 99, 9374-9379
9374
Conformational Analysis. 21. The Torsional Problem in Oxalyl Chloride. An ab Initio and Electron-Diffraction Investigation of the Structures of the Conformers and Their Energy and Entropy Differences Donald D. Danielson? Lise Hedberg? Kenneth Hedberg,*g' Kolbjwn Hagen,' and Marit Traetteberg*a Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, and Department of Chemistry, University of Trondheim AVH, N-7055 Dragvoll, N o w a y Received: March 2, 1995@
The structure and conformational properties of oxalyl chloride, which experiences internal rotation about the C-C bond, have been reinvestigated by electron diffraction from the gas at 0,80, and 190 "C and by extensive ab initio calculations. Complete structure optimizations at a very high level (MP2/TZ2P, 166 basis functions) revealed, in addition to the anti form at LClCCCl = 180", a second stable form (gauche) with LClCCCl = 89.8" characterized by a very shallow minimum in the energy; earlier theoretical results for oxalyl chloride had been inconsistent with the existence of a second form known from experiment to be present. The electron diffraction analysis was based on dynamic models that comprised a set of pseudoconformers spaced at regular intervals around the torsional coordinate @ = LClCCCl and Boltzmann weighted according to a three-term torsional potential V ( @ )= I/2ClV1[ 1 - cos i( 180 - @)I. For the more elaborate model results from the ab initio calculations were incorporated in the form of distance and angle differences among the pseudoconformers; in a second, simpler model these differences were omitted so that the structures of the pseudoconformers differed only in their torsion angles. A theoretical force field for the anti form was also evaluated ab initio, scaled to fit the observed wavenumbers, and used in each model to calculate the usual corrections for vibrational averaging. The results for the analysis of the 0 "C data for the more elaborate (preferred) model are as follows (rg/& L d d e g with 2 0 uncertainty estimates): r(C=O) = 1.184(2), r(C-C) = 1.548(8), r(C-C) = 1.749(3), LCCO = 123.8(4), LCCCl = 111.8(3), and LClCCCl,,,t,, = 76(18) where 0" corresponds to cis; results at the other temperatures are similar. Values of the potential constants, which should be temperature independent, are found in the ranges (kcaymol) 1.45 IVi I1.99, -0.40 d V2 d 0.03, and 0.43 IV3 I1.05;the average values are V I = 1.59(83), V2 = -0.1 1(38), and V, = 0.74(39). The estimated mole fractions of the anti form at 0, 80, and 190 "C are 0.67, 0.62, and 0.43, from which the internal energy - is calculated to be 0.75(50) kcal mol-' and the entropy difference ASo = $ R difference AUo = In 2 - to be 1.31(148) cal mol-' K-I. The simpler model gives similar results.
+
Introduction Investigations of the molecular structure of oxalyl chloride are many in number and span a period of over 40 years. Interest in the molecule centers on the ease with which intemal rotation can occur around the conjugated carbon-carbon single bond, thereby affording the possibility for generation of more than one conformer as suggested by Figure 1. By 1970 it had become clear from spectroscopic and X-ray work that the molecule assumed an anti conformation (symmetry C2h) in the crystal, but in fluid phases certain differences in the spectra indicated the presence of an additional conformer.' This conformer was widely thought to be a planar syn form (symmetry C2"), but in 1973 it was shown by our electrondiffraction study of the gas2 to be gauche with a torsion angle ClCCCl of about 65" (anti = 180') and to be less stable than the anti by about 1.4 kcdmol. These conclusions were verified by a subsequent unpublished study3 of the molecule based on a more elaborate model, one in which the conformational distribution was assumed to be determined by a torsional potential of the form 2V(r)) = CiV;[l - cos i(180 - r))] with i = 1,2, and 3. Among the refined parameters were the constants
' Present address: 2 @
Intel Corporation, Aloha, OR. Oregon State University. University of Trondheim. Abstract published in Advance ACS Abstracts, May 15, 1995.
anti
gauche
Figure 1. Model of cbnfomers of oxalyl chloride.
Vi. In addition to the potential minimum for the anti at r) = LClCCCl = 180", a second minimum was found at r) equal to about 86". From the experimental side there has thus seemed to be no doubt about the existence of a second conformer in gaseous oxalyl chloride or about its identity as gauche. Recently, however, an extensive ab initio investigation4of the molecule opened the question anew. This work comprised a detailed investigation of the basis set dependence of the torsional potential. Structure optimizations were carried out at the HF level with bases ranging from 3-21G* to TZ2P over the range 0" 5 LClCCCl 5 180". A number of single-point MP2/HF calculations were also done, as well as one series of optimiza-
0022-365419512099-9374$09.0010 0 1995 American Chemical Society
Torsional Problem in Oxalyl Chloride tions at the MP2/DZP level. Surprisingly, only the compact HF/3-21G* and the HF/DZ((P)) (polarization functions only on the chlorine atom) resulted in stable second conformers: from the former a planar syn form and from the latter a gauche form with a torsion angle of about 60" from planar syn. Nearly all the results, however, showed at least a dip in the potential in the approximate range 70" 5 LClCCCl I 110" that suggests some kind of stabilizing force for a gauche type of conformer. Since the theoretical torsional potentials, at least those from the higher level calculations, tend to have values less than about 1.Okcal mol-' in this region, these results also suggest that the (anti) molecule undergoes large-amplitude torsional motion. The contrasting pictures provided by the experimental results (two stable conformers) and the theoretical ones just cited (only one stable conformer) has led us to reexamine the electron diffraction aspect of the work. The point of interest is whether the limitations of the models used for our previous experimental work could have led to the wrong conclusion about the conformational composition. The first of these models was built on the assumption that two conformers were present, and the structures of these conformers differed only in the torsion anglesS2It was recognized that the latter part of this assumption could not be wholly correct, but any reasonable deviations from it seemed quite unlikely to affect conclusions about the identity of the conformers and their energy difference. Nevertheless, to the extent that the theoretical potentials might have merit, the two-conformer model could not by its nature represent a fair test of them because the parameters of this model would necessarily have accommodated themselves to give the best possible fit to the data. The second (cosine potential) model3 provided a better test, both because the assumed presence of two conformers was dropped and because estimates of the effects of vibrational averaging ("shrinkage") were included. However, it also contained an assumption that could be viewed as weakening the experimental case for the existence of two stable conformers in that no allowance had been made for the changes in the bond lengths and bond angles indicated by the ab initio results to be a consequence of torsion angle change. We emphasize that other features of both models made them quite sophisticated ones for the time, and both gave excellent fits to the data from experiments at three temperatures. In view of the widespread use of ab initio calculations for structure prediction, it seemed worthwhile to test against our diffraction data a model incorporating additional assumptions drawn from the theoretical data. This model would be based on the three-term cosine potential described above, but the pseudoconformers would include the changes in the parameters obtained by comparison of ab initio optimized structures at various fixed values of the torsion angle ClCCCl. Details of this model and the results obtained from it are the subjects of this paper. Experimental Section
The diffraction photographs were those on which the original study was based;2 the experimental conditions are described there. The data were obtained by retracing3 the plates used in the published work.* (The principal reason for the retracing was a desire to make use of improved photometric equipment.) Instead of the hand-drawn backgrounds from the earlier work, computer-generated ones were removed and the data processed by our usual method^.^ The electron-scattering amplitudes were taken from tablesS6In addition to the data from the three original sample temperatures (0, 80, and 190 "C), additional data were obtained at still higher temperatures, 405 and 525 O C 3 The new high-temperature data were intended to improve the
J. Phys. Chem., Vol. 99, No. 23, 1995 9375 reliability of the conformational energy measurements, but unfortunately extensive sample decomposition occurred at both higher temperatures and the data were found unsuitable for inclusion in the new
Design of the Model Any model of a system of molecules responsible for an electron-diffraction pattern necessarily is built on certain assumptions that may carry important consequences for some of the parameter measurements. For molecules such as oxalyl chloride wherein the main matter of interest is the torsional potential function, it has been our practice to represent this potential by a series of the type 2V(4) = CiVi[ 1 - cos i( 180 4)], with i = 1, 2, and 3, and to represent the system by a set of pseudoconformers chosen at suitable intervals of 4 each weighted by a Boltzmann factor determined by the potential.' The problem to be faced in the design of the more elaborate model was the selection of appropriate additional assumptions concerning the structures and vibrational properties of the pseudoconformers. We decided to construct a model (henceforth model A) invoking certain results of ab initio calculations for the former and the results of such calculations coupled with normal coordinate analyses for the latter. It was done as follows. A set of pseudoconformers was defined at suitable intervals of the torsion angle LClCCCl, and the structures were optimized with use of the programs GAUSSIAN 908 and SPARTAN9 as described in the following section. The normal frequencies and Cartesian force constants were then calculated for the optimized anti form. These were converted to symmetrized internal force constants and scaled to fit the observed wavenumbers'O with a new version of the program ASYM20.I' Using the set of force constants derived for the anti form and omitting the very lowfrequency torsional mode, the root-mean-square amplitudes, perpendicular amplitude corrections, and centrifugal distortions were calculated with ASYM20 for the molecular frames of each pseudoconformer.'* (Details and interpretations of these results will be presented in a future article.) The structure of each pseudoconformer was defined in terms of the parameters of the anti conformer (4 = 180°), to which were added the dzj'erences between parameter values for the form in question and the anti form obtained from comparison of the optimized ab initio structures. The vibration properties of each pseudoconformer were defined in a similar fashion: each distance was assigned an amplitude determined by the difference between its calculated value and that calculated for the anti form. As a result of this procedure, the structures and vibrational amplitudes of the entire system were defined in terms of these properties for the anti conformer. The parameters to be fitted for this model A of the oxalyl chloride system were chosen as the distances r(C=O), r(C-C), r(C-Cl); the angles L(C-C=O) and L(C-C-Cl); several groups of vibrational amplitudes; and the potential constant VI, V2, and V3 in the threeterm potential function. Thirteen pseudoconformers spaced at 15" intervals over the range 0" I4 5 180" were included. The structures were defined in r, space wherein distances are free from the effects of harmonic vibration ("shrinkage"), and the usual corrections to r, and rg were a~p1ied.l~ In the introductory paragraphs we have referred to our unpublished reanalysis of the oxalyl chloride data. The model for this work will henceforth be designated B. Model B differs from A principally in that the bond distances and bond angles of the pseudoconformers were assumed to be unaffected by torsion. Minor differences in the way the frame amplitudes for the pseudoconformers were estimated and assigned also exist.
5h
Danielson et al.
9376 J. Phys. Chem., Vol. 99, No. 23, 1995 4
6 , --C HF/531G'
3.5
+HF/6-311+G* HFlTZlP -*- MP2/6-31G' - e - MP2/6-311+G* - MP2/TZ2P -A-
3 2.5
0)
I
4
'-c CCCl -cco
3
+OCCO - ClCCCl
Q
*-
F 2 @ P I
2 1.5 1 0.5 0
0
30
60
90
120
150
180
