J. Phys. Chem. 1980, 84, 1767-1771
interstellar lines, cases b-d in Tables I1 and 111, results in u 5 2 kHz which is small compared to the uncertainties (except for J = 2) recorded in Table I. When the interstellar frequency 78234.7 MHz is replaced by its alternative velocity component at 78232.1 MHz, Q increases significantly. This change reflects the fact that the former frequency has a larger error associated with it; consequently, it has a lower weight in the fit (see eq 2a). When we focus our attention on the calculated errors (in parentheses) in the GSC in Table 111, we note that the addition of six or seven interstellar lines to the ten laboratory lines produces some significant changes. The errors for to,bo, co, and do decrease in cases b and c. Thus, it appears that the interstellar data facilitate the determination of the higher-order GSC. This may be expected because the observed interstellar transitions are predominantly at higher J than the laboratory transitions. In the abstract, we have quoted the GSC from case c as it has the smaller u for the two data sets which include all laboratory and all interstellar lines. When the GSC of case a are used to calculate all 17 transition frequencies, the resultant a = 1194 Hz; this value is larger than u = 1036 Hz for case C.
We have already compared, in Table 111, our calculated GSC with those determined from microwave laboratory datal4 alone. The corresponding values, with their calculated standard deviations, obtained from infrared laboratory data3 alone are to = -43425 f 69, po = 5.300 f 0.378, and to= -0.4803 f 0.0200 Hz. The higher-order GSC bo, co, and do were not determined in ref 3. In that work, the standard deviations for to, po, and toare considerably greater than in ref 1 4 or the present work. References a n d Notes (1) See, for example, G.Herzberg, "Infrared and Raman Spectroscopy of Polyatomic Molecules", Van Nostrand, Princeton, 1945, p 41. (2) K. T. Hecht, J . Mol Spectrosc., 5, 355, 390 (1960). So-called "forbidden" transitions are usually relatively weak compared to "allowed" transitions; they are not absolutely forbidden, with zero
1767
intensities. G. Tarrago, M. Dang-Nhu, G. Poussigue, G. Guelachvili, and C. Amiot, J . Mol. Spectrosc., 57, 246 (1975). L. W. Pinkley, K. Narahari Rao, M. Dang-Nhu, G. Tarrago, and G. Poussigue, J . Mol. Spectrosc., 63, 402 (1976). K. Fox, Phys. Rev. Lett., 27, 233 (1971). J. K. G. Watson, J. Mol. Spectrosc., 40, 536 (1971). M. R. Aliev, JETPLett. (Engl. Trans.), 14, 417 (1971) [ Z h . Eksp. Teor. Fiz., Pis'ma Red., 14, 600 (1971)]. I. Ozier, Phys. Rev. Lett., 27, 1329 (1971). For a review, see for example, T. Oka in "Molecular Spectroscopy: Modern Research", Vol. 11, K. Narahari Rao, Ed., Academic Press, New York. 1976. DD 229-253. For a review, see {or example, M. R. Aliev, Prog. Phys. Sci., 119, 557 (1976); in Russian. For a revlew, see for example, K. Fox in "The Slgnificance of Nonlinearity in the Natural Sciences", A. Psrlmutter and L. F. Scott., Ed., Plenum Press, New York, 1977, pp 265-292. These "forbidden" transitions are generally much weaker than those alluded to In ref 2. K. Fox, Phys. Rev. A , 6, 907 (1972). C. W. Holt, M. C. L. Gerry, and I. Ozier, Can. J. Phys., 53, 1791 (1975). K. Fox and D. E. Jennings, Bull. Am. Phys. Soc., 23, 318 (1978); Astrophys. J . Lett., 226, L43 (1978). D. E. Jennings and K. Fox, manuscript in preparation. K. Fox and D. E. Jennings in "Proceedings of IAU Symposium No. 87 on Interstellar Molecules", B. H. Andrew, Ed., Reldel, Dordrecht, 1980. F. Mlchelot, J. Moret-Bailly, and K. Fox, J. Chem. Phys., 60, 2606 (1974). The formulation in eq 1 applies equally well to molecules of tetrahedral XY, symmetry, like CH, and octahedral XY8 symmetry, llke SF8. As the present work is on methane only, we have specialized the notation to that of the crystallographic point group Td. Note that the interstellar frequency near 78.2 GHz has two entries in Table I.These correspond to dlfferent observed velocity components, as discussed in detail in ref 16. Both possibilities are included in our calculations. In Table 11, the only values given for the OSD, u, correspond to normallzed welghts defined In eq 2a-c. For each data set, the value of u computed for the unweighted fit is much greater than that for the weighted one. For example, for the second data set in Table 11, u(unweighted) = 37751 compared to u(weighted) = 799 Hz. Different notations are used for the GSC. The relatlonships between those in ref 14 and in eq 1 and Table 111here are eo = -17,.(3/7)'/*/2~ p o = -H,T(3/7)'/2/2 bo = -L,d3/7)1'2/2 Ea = -H,116(2)"*, c = -LeT/16(2)' *, and do = L8T/66(390)'/d.
Microwave Spectrum and Structure of Pyrrole-2-carbonitrile R. Wellington Davis and M. C. L. Gerry* Department of Chemistty, The University of British Columbia, Vancouver, British Columbia VBT 1WS, Canada (Received August 14, 1979)
Microwave spectra in the frequency region 26-75 GHz have been obtained for two isotopic species of pyrrole-z-carbonitrile. Rotational constants and centrifugal distortion constants have been obtained for both species. The molecule has been shown to be planar. Strong evidence is presented for distortion of the pyrrole ring at the substituted C2 atom.
Introduction The synthesis of pyrrole-Z-carbonitrile (I) was first re-
I
ported by Anderson,] and since then improved methods have been reported.2J It has recently been found as part of the neutral fraction of tobacco smoke condensate^.^ 0022-3654/80/2084-1767$01 .OO/O
There has been only a little spectroscopic study in both the and ultraviolet6g7regions. The electric dipole moment has, however, been measured by dielectric methods and has been found to be equal in magnitude to the vector sum of the dipole moments of pyrrole and benzonitrileS8 Pyrrole itself is one of a series of heterocyclic molecules with considerable aromatic character. There is much evidence for this, such as its planar structure, the close similarity of its nominally single and double CC bond lengths (1.417 and 1.382 A, respectivelyg), its ability to undergo
0 1980 American
Chemical Society
The Journal of Physical Chemistry, Vol. 84,
1768
No. 14, 1980
Davis and Gerry
TABLE I: Spectroscopic Constants and Principal Moments of Inertia of Pvrrole-2-carbonitrile parameter
pyrrole-2carbonitrile-1-h
b
HtI
pyrrole-2carbonitrile-1-d
Rotational Constants, MHz
A B
8946.6297(88)a 1988.5936(21) 1626.5440(16)
C
L
8367.099(11) 1988.4938(19) 1606.2658(16)
1373
I
CG
N
Quartic Centrifugal Distortion Constants, kHz AJ AJK
0.0690(26) 2.812(11)
0.0839(33) 2.734(13)
AK
indetb 0.01 69( 26)
indet
SJ SK
1.26(10)
I Y
0.0150(12) 1.461(81)
H
56.4882 254.1 389 310.7072
Ia Ib
4
A = I,
- I,
- Ib
0.0801
60.4007 254.1 5 1 7 314.6297 0.0773
i!
H I
Principal Moments of Inertia, k.&
,
Figure 1. Orientation of the principal inertial axes in pyrrole-2carbonitrile- 1-h. The numbering of the carbon atoms is given. The ring and substituent bond lengths (in A) are those of model f, Table 111, and show the shortening of the ring bonds at the substituted C2atom.
Numbers in parentheses are standard errors in units A K was indeterminate and omitted from the fits.
foundland. Pyrrole-2-carbonitrile-1-d (deuterated a t the ring N) was prepared by exchange with D 2 0 in the microwave cell.
substitution rather than addition reactions, and its very low base dissociation constant.l0 Though its bond lengths and angles are all accurately known,gno attempt has been made to determine the effect of a substitution on the ring structural parameters. Detailed studies of this effect have, however, been conducted on three six-membered aromatic molecules, namely, benzonitrile,ll fluorobenzene,12 and 2,6-difl~oropyridine.'~ In all three cases the ring was found to be pushed in at the substituted carbon atom with shortening of the ring bonds attached to this atom. Naively one might expect this to occur for substituted five-membered aromatic compounds. Though there has been no previous microwave investigation of pyrrole-Zcarbonitrile, the spectra of three closely related molecules have been reported. These are furanand 2-carbonitrile,14J5 thiophene-2-~arbonitrile,'~J~ thiophene-3-carbonitri1e.l' In each case the spectrum of only one isotopic species was obtained, giving hardly enough data to do a definitive structural study. Suggested structures were evaluated, however. For furan-2-carbonitrile it was felt that the rotational constants were best accounted for with a model in which the furan ring was the same as in furan itself, but with the C - C e N chain possessing a bend a t the central C of some 13" toward 0. Several possible structures were suggested for thiophene2-carbonitrile, with a structure containing a distorted thiophene ring considered the most probable. In addition, in thiophene-3-carbonitrile an asymmetric distortion of the thiophene ring was considered very likely. The present study has been carried out to see what structural information could be obtained for pyrrole-2carbonitrile, in spite of the several structural interpretations for the related molecules. It is, however, easy to obtain two isotopic species in the present case, so that considerably more information is available which should narrow the possibilities.
Observed Spectrum and Analysis Initially the spectrum was predicted by using rotational constants calculated from an assumed structure made up from an undistorted pyrrole ring with CN as in benzonitrile attached at the 2 position. Since the dipole moment p had a magnitude calculable by taking the vector sum of the pyrrole and benzonitrile dipole moments, it was assumed that this also gave the direction of the dipole moment, 1.5 D. Accordingly, strong giving pa 3.4 D and hb a-type and much weaker b-type transitions were predicted. This general pattern was indeed what was found. However, in spite of the large value of p, the spectrum was rather weak because of the low volatility of the molecule. The a-type groups were very distinct and had partially resolved K, structure. Although the lines were quite broad (-1 MHz), they could not be split to give 14Nquadrupole coupling constants. Some transitions of vibrationally excited molecules were also observed but no analysis was carried out. The rotational constants B and C were accurately obtained from the measured transitions, along with an approximate value for A. The b-type transitions required to give an accurate value for A were weaker and more difficult to observe, but could be predicted accurately at low K, by using a rigid rotor approximation. Predictions of higher K, transitions were made with the inclusion of quartic distortion effects. A linear least-squares analysis for three rotational constants and five quartic centrifugal distortion constants was carried out. Watson's reduced Hamiltonian18 in its A reduction, P representation, was used, and the analysis used a rigid rotor basis. Because the distortion contributions were very small only first-order contributions were needed. The resulting spectroscopic constants are in Table I. Clearly good values have been obtained for A , B , C, A,, and AJK. The values of aJ and aK are more poorly determined. AK is indeterminate and was omitted from the fits. Table I1 contains the measured transition frequencies and assignments, along with the frequencies calculated by using the spectroscopic constants of Table I, and the distortion contributions.
a
of the last significant figures.
Experimental Section The spectra were measured with a conventional 100-kHz Stark-modulated spectrometer in the frequency range 26-75 GHz. Because of the low volatility of the molecule (vapor pressure 1.5 torr at 90 "C) the spectra were obtained at room temperature. Even then the samples were difficult to handle and move about in the vacuum line. The sample of pyrrole-2-carbonitrile was kindly provided by Dr. Hugh Anderson of Memorial University of New-
-
-
Molecular Structure Table I cogtains, besides the spectroscopic constants, the effective principal moments of inertia and inertial defects of the two isotopic species. Since the two inertial defects are small positive numbers, and are essentially
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
Structure of Pyrrole-2-carbonitrile
1709
TABLE I1 : Observed and Calculated Transition Frequencies (MHz) of Pyrrole-2-carbonitrile frequency I
transition
obsd
calcd
Pyrrole-2-carbonitrile-1-h 27901.48 27901.45 8(0,8)-7(0,7) 8(1,8)-7(1,7) 27236.27 27236.25 28987.13 28987.17 8(5,4)-7(5,3) 28971.32 28971.26 8(6,3)-7(6,2) 9(0,9)-8(0,8) 31166.92 31166.88 30578.96 30579.02 9(1,9)-8(1,8) 9(2,8)-8(2,7) 32290.22 32290.15 9(2,7)-8(2,6) 33654.19 33654.25 9(3,6)-8(3,5) 32841.47 32841.54 9(6,4)-8(6,3) 32602.67 32602.68 9(7,2)-8(7,1) 32588.98 32589.04 32580.10 32579.95 9(8,2)-8(8,1) lO(0,l.O)-9(0,9) 34403.31 34403.32 1 0(1,lO)-9 (1,9) 3 390 6.8 7 3 39 06.80 35807.39 35807.44 10(2,9)-9(2,8) 36588.96 36589.04 10(3,7)-9(3,6) 36237.61 36237.64 10(6,5)-9(6,4) 36219.05 36219.10 10(7,4)-9(7,3) 10(8,2)-9(8,1) 36206.85 36206.83 361 98.1 2 36198.1 0 10(9,2)-9(9 , l ) l l ( 0 , l l ) - l O ( 0 , l O 3’7624.36 37624.25 l l ( 1 , l l ) - l O ( 1 , l O 37220.70 37220.79 ll(2,lO)-10(2,9) 39303.87 39303.79 11(10,2)-lO(l0,l) 39816.01 39815.95 57988.12 57988.12 16(9,7)-15(9,6) 16(11,6)-15(11,5) 67945.1 6 57945.16 17(1J6)-16(1,15) 60735.24 60735.24 17(2,16)-16(2,15) 69811.09 59811.10 17(3,15)-16(3,14) 61604.45 61604.47 17(4,14)-16(4,13) 62067.67 62067.67 62668.34 61668.27 17(8,9)-16(8,8) 17(9,9)-16(9,8) 61628.39 61628.39 17(10,7)-16(10,6) 611599.45 61599.40 17(11,6)-16( 11,5) 61 577.30 61577.36 17(12,6)-16(12,5) 61559.91 61559.96 18(8,11)-17(8,10) 66318.82 65318.78 18(9,10)-17(9,9) 65271.66 65271.60 18(12,6)-17(12,5) 6!5191.01 651 91.1 2 18( 13,5)- 17(13,4) 6 51 74.61 6 517 4.56 18(14,5)-17(14,4) 65160.87 65160.70 18(16,3)-17(16,2) 65138.16 65138.26 20( 9 , l l ) - 1 9 ( 9,lO) 7 2567.54 7 2567.51 20(11,10)-19(11,9) 72485.95 72486.02 20(13,7)-19(13,6) 72436.62 72436.64 20(16,5)-19(16,4) 72389.20 72389.21 lO(1,lO)-9(0,9) 36018.76 36018.78 11(1,11)-10 (0,lO) 38836.1 3 38836.25 18(2,16)-18(1>17) 34626.69 34626.71 14(3,11)-14(2,12) 28153.62 28153.58 15(3,12)-15(2,13) 27514.39 27514.54
distortion corr
dev
-0.09 -0.10 -1.27 -1.79 -0.12 -0.13 -0.34 -0.41 -0.64 - 2.04 - 2.72 -3.51 -0.17 - 0.18 - 0.41 -0.76 -2.31 -3.06 - 3.94 - 4.92 -0.23 -0.23 -0.49 -6.68 -8.41 - 12.13 - 1.31 -1.28 -1.87 - 2.61 -7.37 -9.05 -10.93 - 13.01 -1 5.28 -7.94 -9.72 - 16.31 - 18.93 -21.75 -28.03 - 11.12 - 15.77 -21.35 -31.47 -0.28 - 0.34 -3.44 -2.66 - 3.00
0.03 0.02 -0.04 0.06 0.04 -0.06 0.07 -0.06 -0.07 -0.01 -0.06 0.15 -0.01 0.0 7 -0.05 -0.08 -0.03 -0.05 0.02 0.02 0.11 -0.09 0.08 0.06 0.00 -0.00 - 0.00 -0.01 -0.02 0.00 0.07 -0.00 0.05 - 0.06 - 0.05 0.04 0.06 - 0.11 0.05 0.17 -0.10 0.03 - 0.07 -0.02 -0.01 -0.02 - 0.1 2 -0.02 0.04 -0.15
frequency
equal, the molecule, like pyrrole itself, furan-2-carbonitrile, and thiophene-2-carbonitrile,is planar. Consequently, for the purposes of structural determination, there are only two independent principal moments of inertia for each species, which can be taken as I, and Ib It is noteworthy too that only I, changes appreciably with deuteration, implying that this atom is located close to the b axis. Any reasonable model calculation will show this; it will show also that the C - C r N chain is near the a axis, so that I , is rather insensitive to the positions of these atoms. An attempt has been made to obtain further structural information from the available rotational constants, and in particular by reproducing the observed constants with reasonable structures. Since there are some 21 structural parameters, but only four rotational constants, there is clearly an infinite number of possibilities. However, by making some reasonable constraints sensible structural deductions can be made. Since detailed structures of pyrroleg and of several nitriles, particularly benzonitrilel’ and a~rylonitrilel~ (which
transition
obsd
calcd
16(3,13)-16( 2,14) 17(3,14)-17(2,15) 18(3,15)-18( 2,16) 21(3,18)-21(2,19) 20(4,16)-20(3,17) 24(4,20)-24(3,21)
271 81.59 27220.06 27683.19 31964.44 38748.85 35856.25
271 81.59 27220.13 27683.16 31964.39 38748.75 35856.26
Pyrrole-2-carbonitrile-1-d 27563.93 27563.92 26948.86 26948.87 29895.12 29895.20 28555.44 28555.35 29745.92 29745.97 28838.31 28838.37 28819.23 28819.22 28807.68 28807.60 30769.50 30769.41 30246.60 30246.55 33485.84 33485.90 32433.73 32433.75 32417.48 32417.38 33528.09 33528.06 36052.40 36052.57 36030.37 36030.28 1 0 ( 8 , 2 j - 9 ( ~ ;j1 36015.63 36015.62 l l ( 0 , l l ) - l O ( 0 , l O ) 37126.04 37125.84 ll(2,lO)-10(2,9) 38985.26 38985.30 11(7,5)-10(7,4) 39646.57 39646.65 11(9,3)-10(9,2) 39613.76 39613.77 12(1,12)-11(1,11) 40049.90 40049.88 17(3,14)-16(3,13) 63918.65 63918.60 17(8,10)-16(8,9) 61367.92 61367.98 17 ( 11,6 )-16 ( 11,5) 612 5 8.9 6 6 12 5 9.04 17(12,6)-16(12,5) 61 238.63 61 238.48 18(3,16)-17(3,15) 64642.45 64642.49 18(11,7)-17(11,6) 64876.88 64876.98 18(14,5)-17( 14,4) 64817.27 6481 7.23 19(1,19)-18(1,18) 62640.28 62640.22 11(0,11)-lo( 1 , l O ) 35924.38 35924.44 1 2 ( 0 , 1 2 ) - l l ( 1 , l l ) 39428.88 39429.03 18(3,15)-18(2,16) 26665.07 26665.06 19(3,16)-19(2,17) 28236.89 28236.81 20( 3,17)-20( 2,18) 30346.91 30346.90 16(4,12)-16(3,13) 39459.68 39459.59 18( 4,14)-18 ( 3,15 ) 36 5 3 3.5 1 36 5 3 3.61 20(4,16)-20(3,17) 34097.00 34096.89 21(4,17)-21(3,18) 33327.40 33327.55 23(4,19)-23(3,20) 33131.30 33131.31 24(4,20)-24(3,21) 33831.51 33831.49 25( 4,21)--25 (3,22) 3 51 18.08 35118.12 2 7 (4,23 )-2 7 ( 3,24 ) 394 9 3.81 3 9 49 3.8 2
distortion corr
dev
- 3.36 - 0.00 - 3.73 -0.07 - 4.13 0.03 -5.52 0.05 - 7.52 0.10 -9.76 - 0.01
-0.11
0.01
- 0.1 2 - 0.01
-0.08 0.09 - 0.05 - 0.06 0.01 0.08 0.09 0.05 -0.06 - 0.02 0.10 0.03 -0.17 0.09 0.01 0.20 - 0.04 -0.08 - 0.01 0.02 0.05 - 0.06 - 0.08 0.1 5 -0.04 -0.10 0.04 0.06 -0.06 -0.15 0.01 0.08 0.01 0.09 -0.10 0.11 -0.15 - 8.81 -0.01 -9.26 0.02 - 9.71 - 0.04 - 10.7 0 - 0.01 -0.21 -0.29 -0.35 -1.28 -1.78 -2.37 -0.16 -0.17 -0.28 -2.04 -2.71 - 0.23 -2.31 - 3.06 -3.92 -0.30 -0.53 -3.42 -5.43 - 0.39 - 2.27 -7.45 - 13.00 - 15.24 -2.28 -13.92 - 21.65 -1.55 - 0.21 - 0.30 -4.04 -4.43 - 4.85 -5.07 - 6.22 -7.33 - 7.85
show similar hybridization to pyrrole-2-carbonitrile), are known, we have taken as starting points structures in which the pyrrole ring is unchanged, and the exocyclic C-CEN chain has the bond lengths and angles of these two nitriles. An immediate problem is encountered in such a procedure, however, in that the principal moments are effective moments, whereas the structural parameters are substitution values. By definition the substitution coordinates should not reproduce the effective moments, and small corrections were made to the effective moments to account for this. Because the C - C r N chain is very close to the a inertial axis, and because pyrrole-2-carbonitrile is inertially very similar to benzonitrile, the corrections have been taken to be the same as those of benzonitrile.’l Table I11 contains a comparison of the experimental and calculated rotational constants for the two isotopic species, listed according to various “models”. Model a contains the effective ground rotational constants, and model b gives the “substitution” values described above. Models c and d have been calculated by assuming the ring has the same
1770
The Journal of Physical Chemistry, Vol. 84,
No. 14,
TABLE 111: Comparison of Experimental Rotational Constants (MHz) of Pyrrole-2-carbonitrile with those Calculated from Assumed Structures
modeleh a b C
d e f g h
pyrrole-2carbonitrile- 1-h
pyrrole-2carbonitrile-l-d
A
A
B
Davis and Gerry
1980
B
Experimental Values 8946.63 1988.593 8367.10 9000.16 1992.882 8413.91
1988.494 1992.782
Calculated Values 9024.89 1955.566 8438.38 9024.80 1975.076 8438.18 9024.81 1978.795 8438.17 9003.28 1993.141 8420.42 9027.54 1978.295 8450.49 9022.79 1976.000 8438.51
1955.445 1974.764 1978.683 1993.158 1978.253 1975.909
a Effective ground-state rotational constants. "Substitution" rotational constants, obtained from I," - I," = 0.336 P A ' ; Ibo - Zbs= 0.547 PA'. Ring as in pyrrole. C-C=N as in benzonitrile:" r(C-C) = 1.451 A ; r(C=N) = 1.158 A . Ring as in pyrrole. C-C=N as in acrylonitrile:19 r(C-C) = 1.426 A;r(C=N)= 1.164 A . e Ring as in pyrrole. C-C as in acrylonitrile; C=N as in benzonitrile. Ring: opened at N a n d C, by 0.1" ; C,-C, and C,-N shortened by 0.009 A . Chain: C-C = 1.432 A (cf. 1.426 A in acrylonitrile); C=N = 1.158 A (as in benzonitrile). Ring as in pyrrole. C,-C,=N lengths as in acrylonitrile; chain moved at ring, 2" toward N. Ring as in pyrrole. C,-C,=N lengths as in acrylonitrile; bend in chain at C,, 1.5" toward N.
structural parameters as in pyrrole itself and the C-C=N chain has the same bond lengths as in benzonitrile and acrylonitrile, respectively. Since the uncertainties in the experimental values are probably ,< the differences between the effective and substitution values, apparently neither of the assumed structures reproduces the experimental values, though I, is very close. The acrylonitrile model d is the better of the two. Some changes will nevertheless have to be made to the structures to reproduce the experimental rotational constants. In deducing the adjustments required we have adopted the principle that the most reasonable ones make the smallest possible changes from the above assumed structures. There are five "soft" parameters which have been used for this purpose. These are the following: (i) a ring distortion similar to those of other substituted aromatics, where the ring is squashed and the bonds to the substituted atoms are shortened; (ii) a change in the exocyclic C-C length; (iii) a change in the exocyclic angle of the CN group at the ring; (iv) following the deduction for furan2-carbonitrile,14a bend in the C - C r N chain at the central C; (v) the C=N length. A brief reflection will reveal that these are really the only simple changes possible, with the last two being probably less likely to adjust the calculated constants enough to reproduce the experimental values. In any adjustment to the structure from either the benzonitrile model c or the acrylonitrile model d, the changes necessary must decrease A and increase B , and must be essentially the same for each isotopic species. These requirements can be met by assuming a symmetrical ring distortion similar to those found with other aromatics. Expansion of the ring by 0.1' at C3 and N, an essentially negligible change, gives a sufficient adjustment to A (cf. models f and c or d). Obtaining the desired effect on B is slightly trickier. Although the acrylonitrile model d comes nearest to what is required, the C-C length (1.426 A) is probably a lower limit. One can do no more shortening of this bond to reproduce B. Shortening C=N to its length in benzonitrile (1.158 A, probably also a lower limit) is not enough, as is
TABLE I V : Observed and Calculated Rotational Constants (MHz) of Furan-2-carbonitrile rnodeP
A
B
a b
9220.11 2029.262 9276.97 2033.728 C 9290.05 1999.916 d 9289.81 2020.727 a Effective ground state rotational constants, ref 14. "Substitution" rotational constants, from I,' - IaS= 0.336 P A a \ I b - Ibs =. 0.547 118'. Ring as in furan;" C-C=N as In benzonitrile. Ring as in furan;" C-C=N as in acrylonitrile. e The models parallel exactly those of pyrrole-2-carbonitrile in Table 111.
seen by comparing models d and e. However, if the Cz-C3 and C2-N ring bonds are both shortened by 0,009 A, similar to the six-membered arornatic~,ll-'~ the requirement is met. Model f gives the rotational constants calculated with this ring distortion, r(C=N) = 1.158 A as in benzonitrile, and r(C-C) = 1.432 A (slightly longer than in acrylonitrile). This is clearly a satisfactory model and is shown in Figure 1. It must be remembered that both these C-C and C r N lengths are at or near their lower limits. These bonds may well be longer, implying an even greater ring contraction. The effects of the other reasonable structural changes were also considered. A comparison of models g and d shows the effect of moving the C2-C6=N chain about the ring carbon C2, 2' toward N. (An equal but opposite effect is obtained by the same bend away from N.) The trend in B is reasonable and a bend of 12" would be enough to reproduce the experimental values. However the change in A is in the wrong direction, and furthermore, for the deuterated species it would be some 70 MHz in the wrong direction, considerably more than -20 MHz for the H species. This structural change cannot be the only one for this molecule; if it were it must be accompanied by a ring distortion even greater than that proposed above. A comparison of models d and h shows the effect of bending the C2-C6=N chain 1.5' toward the ring N. In both cases there is an increase in B. However, in order to reproduce B a linear extrapolation suggests that a bend of -25' is required, a most unreasonable value. The change of A for the normal species is in the right direction but not for the deuterated species. Once again this cannot be the only parameter to account for the rotational constants, but a small bend in this chain, in conjunction with other changes, cannot be ruled out.
-
Discussion Although there is an infinity of possible structures which can account for the experimental data, the deductions of the previous section have shown that the simplest model to do so contains a pyrrole ring slightly squashed a t the substituted Cz atom and a linear C - C r N chain with a rather short C-C bond at the same angle as C2-H in pyrrole. Other models cannot of themselves do what is required, though our simple model does not preclude other changes occurring to some degree. We feel, however, that we have presented very strong evidence for a ring distortion similar to those in six-membered aromatics and to that considered most likely for thiophene-2-carbonitrile. Although the structural information was obtained by trying to reproduce the "substitution" rotational constants, model b, it is worthwhile considering what is required to reproduce the effective constants, model a. It is apparent from Table I11 that the same general conclusions must be reached. Expansion of the ring to reproduce A must be even greater than we have shown above. And the re-
J. Phys. Chem. 1980, 84, 1771-1782
quirements to reproduce B are very similar. Finally, in the light of our present deductions, it is interesting to reconsider furan-2-carbonitrile, for which a bend of some 13' in the C-CEN angle was deemed to be rea~onab1e.l~ Table IV contains the rotational constants of this molecule, exactly paralleling models a-d of Table 111. Much the same trends are found for the furan derivative as for the pyrrole derivative. Again model d reproduces the experimental rotational constants better than model c, and in fact, better than model d in the pyrrole derivative. Clearly similar conclusions can be reached for furan-2-carbonitrile as for pyrrole-2-carbonitrile, and extreme structural changes such as the 13' bend in the CC=N chain are not required.
Acknowledgment. We thank Dr. H. J. Anderson for the sample of pyrrole-2-carbonitrile. This work was supported by the Natural Sciences and Engineering Research Council of Canada.
References and Notes (1) H. J. Anderson, Can. J . Chem., 37, 2053 (1959). (2) C. E. Loader and H J. Anderson, Can. J . Chem., 49, 45 (1971). (3) H. J. Anderson, C. R. Riche, T. G. Costeilo, C. E. Loader, and G. H.
1771
Barnett, Can. J . Chem., 58. 654 (1978). (4) H. Shigematsu, R. Ono, Y. Yamashita, and Y. Kaberaki, Agr. Biol. Chem., 35, 1751 (1971). (5) L. H. Deady, R. A. Shanks, and R. D. Topsom, Tetrahedron Left., 1881 (1973). (6) L. F. Elsom'and R. A. Jones, J . Chem. SOC.6 , 79 (1970). (7) T. Marey and J. Arriau, C . R . Acad. Sci. Paris, Ser. C , 272, 850 (1971). (8) D. M. Bertin, M. Farrier, and C. LiBgeois, Bull. SOC.Chim. Fr., part
1, 2677 (1974). (9) L. Nygaard, J. T. Nlelsen, J. Kirchheiner, G. Maltesen, J. RastrupAndersen, and G. 0. Sbensen, J . Mol. Strucf., 3, 491 (1969). (10) See, for example, C. R. Noiier, "Chemistry of Organic Compounds", 2nd ed., Saunders, Philadelphia, 1957. (11) J. Casado, L. Nygaard, and G. 0. Sbensen, J . Mol. Sfruct., 8 , 21 1 (1971). (12) L. Nygaard, I. Bojesen, T. Pedersen, and J. Rastrup-Andersen, J. Mol. Sfruct., 2, 209 (1968). (13) 0.Stiefvater, Z . Naturforsch. A , 30, 1765 (1975). (14) L. Engelbrecht and D. H. Sutter, Z . Naturforsch. A , 31, 670 (1976). (15) J. Wiese, L. Engeibrecht, and H. Dreizler, 2. Naturforsch. A , 32, 152 (1977). (16)T. K. Avirah, T. B. Malloy, and R. L. Cook, J . Mol. Struct., 29, 47 (1975). (17)J. Wiese and D. H. Sutter, Z . Naturforsch. A , 32, 890 (1977). (18) J. K. G.Watson in "Vibrational Spectra and Structure", J. R. Durig, Ed., Elsevier, Amsterdam, 1977,p 1. (19)C. C. Costain and B. P. Stoicheff, J. Chem. Phys., 30, 777 (1959). (20) B. Bak, D. Christensen, W. B. Dixon, L. Hansen-Nygaard, J. Rastrup-Andersen, and M. Schofflander, J. Mol. Spectrosc.,9, 124 (1962).
Vibrational Spectrum, Force Field, and Torsional Potential Function of Monothioformic Acid in the Gas Phases B. P. Winnewlsser" Physikallsch-ChemischesInstitut, Justus-Liebig-Universitat Giessen, 0-6300Giessen, West Germany
and W. H. Hocklngt Max Planck Institute fur Radioastronomie, 0-5300Bonn, West Germany, and Physikallsch-Chemisches Institut, Justus-Liebig-Universltat Giessun, 0-6300Giessen, West Germany (Received July 31, 1979)
The infrared absorption spectra of five isotopic species of monothioformic acid (thiolformic acid), HCOSH, HWOSH, HCl*OSH,DCOSH, and HCOSD, have been measured in the gas phase. As far as possible, the fundamental vibrations of both the cis and trans rotamers have been identified. These data were combined with centrifugal distortion constants from the rotational spectra for the above isotopic species and the 34Ssubstituted species to determine the force field for each rotamer. Some significant effects of isomerization on the spectra and force field were determined, and the barrier to internal rotation about the CS bond was determined. The coefficients of the cosine expansion of the torsional potential are Vl = 239 cm-l= 2.86 kJ/mol and V 2 = 3408 cm-l = 40.77 kJ/mol.
I. Introduction The rotational isomerism of monothioformic acid in the gas phase was established in the study of the microwave and millimeter wave spectrum by Hocking and Winnewi~ser.'-~The barrier to internal rotation in solution has been determined by dynamic NMR measurements, using CD2Clzand CClZFzas solvents.6 Because the two planar rotamers, cis-HCOSM and trans-HCOSH (in which the *Presentedin part at the Thirtyfourth Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, Ohio, June 11-15, 1979, as paper TC'4. 'Anatek Electronics Ltd., Hybrid Facility, 1332 Main St, North Vancouver, VIJ-LC3, Canada. *Address correspondence to this author at the PhysikalischChemisches Institut, Juslus-Liebig-Universitat Giessen, Heinrich Buff-Ring 58, D-6300 Giessen, West Germany. 0022-3654/80/2084-177 lS0l .OO/O
two hydrogens are cis or trans to each other), have a relative abundance of roughly 1:3 in the gas phase a t room temperature, this molecule is well suited for the study of rotameric effects. The pure rotational spectrum yielded the slightly different substitution structures of the two rotamers? a large difference in the dipole moment^,^ and precise relative intensity measurements at room temperature, from which the energy difference, AE,, between the ground states of the two rotamers was determinede3 In addition the quartic centrifugal distortion constants, which depend on the harmonic force field, were obtained for the parent species and each of the five singly substituted isotopic ~ p e c i e s . ~ ~ ~ ~ ~ We decided that the vibrational spectrum of monothioformic acid would allow the study of isomerism because of the small number of fundamental vibrations: seven a', 0 1980 American
Chemical Society
