4238
J . Phys. Chem. 1991, 95, 4238-4241
following interpolation formula describes in a reasonable way the dependence of q on the elongation X. q(s;x) = 0.0070 exp(-gx")/(X
+ 0.698 A)
(12)
As values of n and of g ensuring a rise as fast as expected from orientative computations, we shall take 3 and -1270. Substitution in eq 12 and numerical integration yields the results of Table IV. Thus, after a single vibration the initial state is 97% depleted. If n is taken as I , then the depletion is not so sudden at the end of a half vibration, but will be anyway 75% after a single vibration; this is largely sufficient for physical processes depending on proton transfer to break out. Moreover, in model calculations the barrier of the brid e appears to vanish for bridge lengths much larger that 2.142 ,and therefore the asymptote of eq 12 should probably lie much closer to -0.2 than is assumed in eq 11. If so, the depletion would be much more effective. Thermal excitation, of course, will only make depletion faster, for one thing because the two heavy nuclei will be periodically brought closer to one another than in the lowest energy vibration. Since the frequency is the same, the temporal aspects remain unchanged. (ii) The considerations of section 5 about the role of other degrees of freedom also apply to environmental effects. In addition, the latter should play a minor role in the most interesting cases (H bonds embedded in a macromolecule), as is borne out by observed bond lengths and force constants of H bonds in prot~ins.'~ Especially peptide H bonds are well protected against external perturbations. On the other hand, the present results suggest that special structural features affecting H-bond stretching frequencies might be responsible for the rate at which proton transfer takes place at the reaction sites of certain enzymes.
R
9. Conclusion
The picture emerging from the above analysis is very important, though tentative. Firstly, the oximine form of the peptide bond appears to be a legitimate element of intermediatestructures of protein reactions. Secondly, the clock controlling the oximine form lifetime is the bridge vibration. These statements are open to discussion and require further work, especially by experimentalists. However, their significance does not depend on their being true but on the possibilities of exploration they suggest. The possibility of proton relay via peptide bonds opens up a number of mechanistic possibilities that might explain the way in which enzymes are protected against external perturbations. The idea that the temporal evolution of enzymatic reactions as well as proton transport through membranes is controlled by an internal clock independent of the environment, if nothing else, opens the question of the time regulation of elementary processes in vivo. Of course, one of the first steps along these lines would be to apply the simple picture obtained here not only to the restoration of the status quo ante (return to the amide form) but to the very formation of the active state. This, however, would require consideration of the driving force, which is in itself a separate problem.
Acknowledgment. G.D.R. acknowledges support by Italian CNR and MPI. C.A. thanks IPE (Naples) for a grant. The formamide computations are the starting point of work in progress by D. Hofmann of the Chair for Theoretical ChemistryErlangen. Registry No. Formamide, 75-1 2-7; formamide dimer, 28704-51-0.
Conformatlonal Properties and Gas-Phase Structure of (Fluorocarbony1)sulfenyl Chloride, FC(0)SCI. Electron DMractlon, Vlbratlonal Analysis, and ab Initio Calculations Hans-Georg Mack," Heinz Oberhammer,*.laand C. 0. Della V&lovalb Institut fiir Physikalische und Theoretische Chemie, Universitat Tiibingen, D- 7400 Tiibingen, FRG, and Lehrstuhl fCr Anorganische Chemie II, Ruhr-Universitirt, 0-4630 Bochum. FRG (Received: May 8, 1990: In Final Form: December 28, 1990)
The gas-phase structure and conformational composition of FC(0)SCI were determined by electron diffraction. The following eometric parameters (r, distances and 4 angles with 3u uncertainties) were derived for the trans conformer: C==O = 1.179(4) C-F = 1.342(4) A, S - C = 1.756(5) A, S-CI = 1.996(3) A, C-S-CI = 100.3(5)', S--(3-0 = 130.9(5)', S-C-F = 105.3(3)O. The free-energy differences AGO = Go(cis)- Go(trans) from the electron diffraction experiment (AGo(ED) = 1.2(3) kcal/mol) and from matrix IR spectra (AGo(IR) = 1.4(1) kcalfmol) are equal within their estimated error limits. Ab initio calculations at various levels of theory (HF,MP2, and MP4) reproduce the experiment very well (AE= 1.25-1.60 kcal/mol). Additional calculations for thioformic acid and various fluorine- and/or chlorine-substitutedderivatives reveal a strong substituent effect on the conformational properties.
W,
Introduction The geometric isomerism of thioformic acid, HC(O)SH, and its derivatives has been of considerable interest for many years. Due to internal rotation around the C-S single bond, trans and cis isomers can occur: (1) (a) Univmitat TObingen. (b) Ruhr-UniversitBt. On leave of absence from Faculty of Exact Scienas. National University of La Plata, Repilblica Argentina. Member of the Research Career and postdoctoral fellow of Consejo Nacional de Investigaciones Cienthas y Ttcnicas (Conicet),
RepOblica Argentina.
O\
c-s /
/
H
=
H
O\
,c-s
trans
\
H
H
cis
The relative stabilities depend on the various interactions between single bonds, the double bond, lone pairs, and substituents, and their relative influence on the conformation cannot be predicted a priori. Extensive microwave studies show that the trans conformation of HC(0)SH is energetically preferred with the cis isomer being higher in energy by 0.6613(17) kcal/moL2 This
0022-365419112095-4238302.50/0 0 1991 American Chemical Society
Conformational Properties and Structure of FC(0)SCl
The Journal of Physical Chemistry, Vol. 95, NO. 11, 1991 4239 TABLE I: Results of Electron Diffnctiw A ~ l y s i 8 s d 8b Initio Calculations (HF/6-31C*)
parameter C=O
C-F
s-c
s-CI c-s-CI
S-C=O S-C-F F-Ce
w
0.* .F S** *F
s.* .o c.* .c1 0.* .Cl
F* *C1
nn I l l
IA I
on
n
m o I
,
+an+
0
0
I
1
2
1
3
4
5
RIA
Figure 1. Experimental radial distribution curve, calculated curves for trans and cis isomers, and difference curve for mixture.
energy difference is much smaller than that for formic acid, HC(O)OH, where the experimental energy difference between cis and trans forms is 3.902 kcal/m01.~ The chlorinated species, (chlorocarbony1)sulfenyl chloride, ClC(O)SCI, was investigated by vibrational spectroscopy4 and gas electron d i f f r a c t i ~ n . According ~ to the gas-phase infrared spectra, only the trans conformer is present, whereas the electron diffraction analysis allows for the presence of small amounts of a second conformer (6.5 f 9.9%gauche). The conformational properties of (fluorocarbony1)sulfenyl chloride, FC(O)SCI, were studied by infrared and Raman spectroscopy, and from the observed splitting of the C - 0 vibration the presence of trans and cis planar isomers was c~ncluded.~,’
*
O\ c,-s
0
cs-’,
F
F
trans
/
0.031/61 0.04 1 (6j 0.057(5) 0.058(2)
0.054(7) 0.069(6) 0.072(8) 0.082( 13) 0.126( 16) 0.091(9)
cis
gmm” 1.183 1.329 1.757 2.004 103.8 121.5 114.5 124.0 12(5) 2.2 18 2.604 2.582 2.964 2.881* 4.109’
1.160 1.314 1.770 2.010 100.7 129.9 106.3 123.8 89
1.164 1.301 1.771 2.018 104.2 120.8 115.3 123.9 11
former.
f
I
1.179(4) I .342(4j 1.756(5) 1.996(3) 100.3(5) 130.9(5) 105.3(3) 123.9(6) 88(5) 2.226(8) 2.475(7) 2.680(8) 2.885( 11) 3.077( 18) 4.086( 10)
e.d. amp.b
O r , distances (A) and L, angles (deg). *Values in A with 3u uncertainties. CThesegeometric parameters were not refined; they were obtained from the respective trans values and the calculated differences (HF/6-31G*) between cis and trans parameters. “Dependent parameter. ‘O-..F distance for cis conformer. fO.-.CI distance for cis con-
c
c w O4Qb
trans gmma
ab initio calcn trans cis geom gmm
CL
cis
From the band types in the IR gas-phase spectra, it was concluded that the trans form is by far more abundant at room temperature. Assuming equal absorption coefficients (square of the dipole moment derivatives) for the C - 0 vibrations for both isomers, the temperature dependence of the infrared intensities results in an enthalpy difference of AHo = (0.8(1) kcal/mol for the gas phase. Analysis of Raman spectra lead to a value of 2.0(2) kcal/mol for the liquid state.’ (2) Hoclring, W. H.; Winnewisser, 0.2.Naturforsch. 1976, 31a, 995. Hocking, W. H.; Winnewisser, G.Z.Naturforsch. 1977, 32a, 1108. (3) Hocking. W. H. 2.Narurforsch. 1976, 31a, 1113. (4) Della Vtdova, C. 0.;Cutin, E.H.; Varetti, E. L.; Aymonino, P.J. Con. J . Spectracc. 1984, 29, 69. (5) Shen, Q.;Hagen, K. J. Mol. Srrucr. 1985, 128.41. (6) Della Vtdova, C. 0.; Varetti. E.L.; Aymonino, P.J. Can.J . Spcrrosc. 1983, 28, 107. (7) Della Vtdova, C. 0.; Cutin, E. H.;Jubert, A. H.; Varetti, E. L.; Aymonino, P.J. Can.J . Spcrrosc. 1984, 29, 130.
In this work we present a gas-phase electron diffraction analysis of the structure and conformational composition of FC(0)SCI. This study is augmented by a matrix IR investigation and by ab initio calculations at various levels of sophistication. In addition to the geometries and relative stabilities of the isomers, the vibrational frequencies and barrier to internal rotation were calculated. In order to obtain information about the influence of different substituents at carbon and at sulfur on the conformational properties, the following compounds were included in the calculations: HC(O)SH, FC(O)SH, ClC(O)SH, FC(O)SF, ClC(O)SCl, and ClC(0)SF. Only for HC(0)SH and FC(0)SCl can the calculated energy differences be compared to experimental values, a rough experimental estimate is available for ClC(0)SCl.
Gas Electron Diffraction Analysis Model calculations demonstrate that the experimental radial distribution function (see Figure 1) cannot be reproduced with one single conformer. If bond angles from analogous compounds are used, the isolated peak at ca. 4.1 A can only be fitted with planar conformations. This peak corresponds to the F 4 1 nonbonded distance of the trans isomer or to the O-.Cl distance of the cis isomer. For gauche conformers, the longest nonbonded distances F-Cl or 04!1 would be shorter than 4.0 A. Such structures can be excluded on the basis of the radical distribution curve, where no peaks occur between 3.2 and 4.0 A. Calculated radial distribution functions for the cis and trans conformers are shown in Figure 1. For this purpose the final parameters (Table I) were used. Although oxygen and fluorine have very similar scattering amplitudes the radial distributions show distinctive differences in the range between 2.4 and 3.2 A. The comparison of the two calculated functions with the experimental curve clearly demonstrates the predominance of the trans conformer. The preliminary geometric parameters and relative contributions of the isomers were refined by a least-squares analysis based on the molecular intensities. For this, scattering amplitudes and phases reported by Haasenwere used and a diagonal weight matrix was applied to the intensities. In the first refinements, all geometric parameters and vibrational amplitudes were assumed to be equal for the trans and cis forms. The amplitudes for the O-CI (F-CI) distances in the trans form were set equal to those of the F...CI (0-C1) distances in the cis isomer. Since the ab initio calculetions (HF/6-31G*) predict large differences in the geometric parameters of the two isomers (see Table I), especially for the SCO and (8) Haase, J. Z.Narurforsch. 1970, 250, 936.
4240 The Journal of Physical Chemistry, Vol. 95, No. 11, I991 1.1
I
0.85
1
1
0.77
TABLE II: Optimized Structures of FC(0)SCI for Variow Torsional Ana= around the CS Bond and Relative Jhergks (HF/6-31Go) of Conformers torsion angle, dcg
0.73
paramete? (trans)
T
T
0.9
Mack et al.
0.81
0.0
07 05
C=O
C-F
0.69
s-c 03
0.65 1860
1820
cm-i
1780
Fipn 2. Matrix IR spectrum of C=O stretching (a) before and (b) after UV irradiation.
SCF angles, the calculated differences were introduced as additional constraints into the final refinements. Hereby, the agreement factors for both camera distances decreased and bond angles for the predominant trans conformer changed slightly. The final results are collected in Table I. No correlation coefficient had a value larger than 10.61. Experimental error limits are 3c values, and a possible scale error of 0.1% is included for bond lengths.
Vibrational Analysis Figure 2a shows the N2-matrixinfrared spectrum of FC(0)SCI in the C - 0 stretching region. Two bands are observed, the more intense at 1838 cm-' corresponding to the trans conformer and the less intense at 1798 cm-', to the cis form. If the matrix is irradiated for 6 min with UV light of X +1.6W
+0.6613( 17)b
ClC(0)SF FC(0)SCl +0.39 +1.25 +0.69 +1.60 +0.69
+1.50
+1.3(3)d +1*5(1)*
*A!?= Ed - EtnM,in kcal/mol. 'Reference 2. cReference 5. dhEderived from electron diffraction experiments, see text. e A E derived from IR spectroscopy, see text.
U
0
5
10
15
20
25
30
35
s/A-' Figure 3. Experimental (dots) and calculated (full line) molecular scattering intensitics and differences.
stabilizes the trans forms more, increasing AE between 0.04 kcal/mol in FC(0)SH and 0.85 kcal/mol in HC(0)SF. The only exception is ClC(O)SCl, where AE decreases slightly. In HC(0)SCI these correlation effects change the ground-state conformation from cis to trans. A higher level of correlation treatment (MP4SDTQ) has small effects, in some cases lowering and in other cases increasing AE. Conclusion The gas electron diffraction study results in a mixture of trans and cis conformers of FC(0)SCI with preference of the trans form. The energy difference derived from the electron diffraction experiment agrees with the value derived from matrix IR spectra. Ab initio calculations with double-f basis sets including polarization functions, with or without electron correlation, reproduce this energy difference well for FC(O)SCI, in contrast to thioformic acid, HC(O)SH, where such calculations are at variance with the experiment.
Acknowledgment. We acknowledge financial support by the Fonds der Chemischen Industrie. H.G.M. thanks the Deutsche Forschungsgemeinschaft for a postdoctoral fellowship. C.O.D.V. gratefully acknowledges Prof. Dr.A. Haas (Ruhr-Universitat Bwhum, F.R.G.) for his stimulating contribution and generous support of this wor) and thanks the Consejo Nacional de Investigaciones Cientificas y Tknicas (Conicet). Repiiblica Argentina, for the receipt of a postdoctoral fellowship and for permission (with stipend) to work in Germany from the Faculty of Exact Sciences, National University of La Plata, RepGblica Argentina. Supplementary Material Available: Tables of the average total intensities for FC(O)SCl(2 pages). Ordering information is given on any current masthead page. (14) Haas, A.; Reinke, H. Angew. Chem. 1967, 79, 687. (15) Oberhammer. H. Molecular Structure by Diffraction Methods;The
Experimental Section
(Fluorocarbony1)sulfenyl chloride, FC(O)SCI, was prepared by the reaction between ClC(0)SCl and SbF3."
product was purified by fractional distillation and it was subsequently purified several times by fractional condensation at reduced pressure in order to eliminate volatile compounds. The purity of the compound was confirmed by IR (vapor), Raman (liquid), 19F NMR, and IlC NMR spectroscopies. The electron diffraction intensities were recorded with the Balzers Gasdiffractograph15 at two camera (nozzle-to-plate) distances (25 and 50 cm) with an accelerating voltage of about 60 kV. The electron wavelength was calibrated with a ZnO diffraction pattern. The sample reservoir was held at -35 O C and the nozzle .was at room temperature. The camera pressure did not exceed 5 X lod Torr during the experiment. The exposure times were 5-7 and 20-25 s for the long and short camera distances, respectively. Two plates of each camera distance .were analyzed by the usual methods.16 The averaged molecular intensities in the scattering ranges 2-18 and 8-33 A-I, in steps of As = 0.2 A-' are presented in Figure 3.
The liquid
Chemical Society: London, 1976, Vol. 4, 24. (16) Oberhammer, H.; Gombler, W.; Willner, H. J. Mol. Struct. 1981, 70, 273.
