J . Phys. Chem. 1987, 91, 5194-5202
5194
A Local-Mode Analysis of the Overtone Spectra of Some Monosubstituted Cyclopropanes M. Khalique Ahmed and Bryan R. Henry* Department of Chemistry, Uniuersity of Manitoba, Winnipeg, Manitoba, Canada R 3 T 2N2 (Receiued: February 25, 1987; In Final Form: May 19, 1987)
The CH-stretching overtone spectra of cyclopropyl bromide (CpBr), cyclopropyl cyanide (CpCN), cyclopropylamine (CpAm), cyclopropyl methyl ketone (CpMK), and chloromethylcyclopropane (CMCp) are measured at room temperature in the liquid phase in the regions of AucH = 2-6 and, for CpAm, in the regions of AUNH = 2-6. For CMCp the spectra are also measured = 2-4. The spectra of CpBr, CpCN, CpAm, and CpMK can be understood on the in the gas phase in the regions of hCH basis of contributions corresponding to methylene and methine ring C H bonds. Such independent contributions do not appear in the spectra of CMCp. We successfully analyze the spectra by assuming that the stretching motions of the C H bonds located at different carbon centers are uncoupled. The spectra associated with the methyl C H bonds of CpMK are described by assuming that the unique methyl C H bond in the plane of the carbonyl group is uncoupled to the other two methyl CH bonds. Thus the spectra of these molecules can be understood on the basis of various XH, and C H units. The vibrations of the XH2 moieties are described in terms of a harmonically coupled local-mode model. The local-mode parameters, w and W X , and the coupling parameter, y ' w , for the XH2 units are determined. The values of w and wx for the ring CH bonds are close to those which are characteristic of sp2 hybridized C H bonds, in agreement with currently accepted models of hybridization in cyclopropanes.
Introduction
Overtone spectroscopy and the local-mode model have also been used extensively to study substituent effects on the XH bonds of The spectral features of the XH-stretching (X = C , 0, N, or polyatomic m o I e c ~ I e s . ~ ~ - ~ ~ S) overtone spectra of polyatomic molecules can be understood In our previous work,15 we reported the liquid-phase C H in terms of the local-mode In this model the higher stretching overtone spectra of a variety of cycloalkanes (cycloovertones are dominated by transitions to states whose components pentane to cyclooctane) and cycloalkenes (cyclopentene, cyclohave all of the vibrational energy localized in a single X H bond. hexene, and cycloheptene). In 1982, Wong et a1.I6 investigated The energies of these peaks can be fitted to the energy equation the photoacoustically detected gas-phase spectra of several of these of a diatomic anharmonic Morse oscillator2-5 molecules, and of cyclobutane and cyclopropane. In both of these studies, peaks due to structurally and conformationally nonE = VU - (c2 C ) U X (1) equivalent C H bonds were identified and were assigned through a local-mode analysis of the observed spectra. Here, w and w x are the harmonic frequency and the diagonal In this paper, we report our observations of the liquid-phase local-mode anharmonicity constant, respectively. These paramovertone spectra of several monosubstituted cyclopropanes: cyeters are very sensitive to physical and chemical properties of the clopropyl bromide (CpBr), cyclopropyl cyanide (CpCN), cycloXH oscillator such as bond length/strength,6-I0 hybridization,2 propylamine (CpAm), cyclopropyl methyl ketone (CpMK): and steric effects,2,"*'2 and conformational e n v i r ~ n m e n t . ' . ~ $ ' ~ - ~ ' chloromethylcyclopropane (CMCp). Our four main objectives in this study were to answer the following questions: (i) How strong is the coupling between the CH bonds located a t different carbon centers in the monosubstituted cyclopropanes? ( I ) Henry, B. R. Acc. Chem. Res. 1977, 10, 207. (ii) How successful is the harmonically coupled local-mode (2) Henry, B. R. Vib.Spectra S f r u c f .1981, 10, 269. theory for an XH, system25in assigning and calculating the spectra (3) Child, M. S.; Halonen, L. A h . Chem. Phys. 1984, 57, I . associated with the various X H 2 groups in these molecules? (4) Child, M. S. Acc. Chem. Res. 1985, 18, 45. (iii) To what extent do the substituents cause asymmetry in (5) Sage, M. L.; Jortner, J. A h . Chem. Phys. 1981, 47, 293. the bond strengths of the ring C H bonds? (6) Mizugai, Y . ;Katayama, M. Chem. Phys. Lett. 1980. 73, 240. (iv) How different is the strength and hybridization of the ring (7) Wong, J. S.; Moore, C . B. J . Chem. Phys. 1982, 77, 603. C H bonds of the monosubstituted cyclopropanes compared with (8) Henry, B. R.; Cough, K. M. Laser Chem. 1983, 2, 309. the analogous CH bonds in the larger cycloalkanes? (9) Cough, K. M.; Henry, B. R. J . Phys. Chem. 1984, 88, 1298. For CpAm, CpMK, and CMCp, there is the possibility of (IO) Cough, K. M.: Henry, B. R . J . A m . Chem. Soc. 1984, 106, 2781. rotational isomers. Wurrey et have reviewed the confor( 1 1 ) Henry, B. R.; Miller, R. J . D. Chem. Phys. Lett. 1978, 60, 81. mations of a number of three-membered ring compounds including (12) Henry, B. R.; Mohammadi, M. A,; Thomson, J. A. J . Chem. Phys. 1981, 75, 3165. CpAm, CpMK, and CMCp. In Table 6 of this article,I6 they (13) Henry, B. R.; Greenlay, W. R. A. J . Chem. Phys. 1980, 72, 5516. summarize information on the conformational preferences of these (14) Henry, B. R.; Mohammadi, M. A . Chem. Phys. 1981, 55, 385. molecules based on both previous experimental and theoretical
+
( 1 5 ) Henry, B. R.; Hung, I . F.; MacPhail, R. A,; Strauss, H. L. J . A m . Chem. Soc. 1980, 102, 5 15. (16) Wong, J . S.; MacPhail, R. A,; Moore, C. B.; Strauss. H . L. J . Phys. Chem. 1982, 86, 1478. (17) Fang. H. L.: Swofford, R. L. Appl. Opt. 1982, 21, 5 5 . (18) Fang, H. L.: Meister, D. M.: Swofford, R. L . J . Phj,.~.Chem. 1984, 88, 405. (19) Fang, H. L.; Meister, D. M.: Swofford, R . L. J . Phys. Chem. 1984, 88, 410. (20) Fang, H. L.; Swofford,R. L.: Compton, D. A . C. Chem. Phys Lett. 1984. 108. 539.
0022-3654/87/2091-5 194$01.50/0
(21) Fang, H. L.; Swofford, R. L.; McDevitt, M.; Anderson, A. B. J . P h j ~ Chem. 1985, 89, 225. (22) Gough, K. M.; Henry, B. R. J . Phys. Chem. 1983, 87, 3433. (23) Mizugai, Y.; Katayama, M . J . Am. Chem. So?. 1980, 102, 6424. (24) Mizugai, Y . ;Katayama, M.; Nakagawa, N. J . Am. Chem. SOC.1981, 103, 5061. (25) Mortensen, 0. S.;Henry, B. R.; Mohammadi. M. A. J . Chem. Phys. 1981, 75, 4800. (26) Wurrey, C. J.; DeWitt, 3 . E.: Kalasinsky, V . F. Vib. Specrra Strucr. 1983, 12, 205.
0 1987 American Chemical Society
Spectra of Some Monosubstituted Cyclopropanes studies. For C P A ~ , * ' -94% ~ ~ of the molecules in the vapor phase exist in a conformer (trans) in which the amino hydrogens are trans to the ring C-C bonds. This conformer is stated to be the only one present in the solid state. Although no quantitative results are given for liquid-phase CpAm, the trans conformer is stated to be the most favorable. For CpMK,31-34either cis or trans conformers could exist, depending on whether the carbonyl and cyclopropyl groups lie in the same side or opposite sides of the connecting single bond. The relative populations of the cis and trans conformers in the gas phase are 80% and 20%, respectively. In the solid state, CpMK exists totally in the cis form, and in the liquid phase the cis conformer, is the most favorable. For C M C P , ~both ~ cis and gauche conformers could exist. From studies of the fundamental I R and Raman spectra, Kalasinsky and W ~ r r e yhave ~ ~concluded that C M C p exists as an equilibrium mixture of gauche (95%) and cis ( 5 % ) in the liquid phase, and that the only conformer in the solid phase is the gauche. XH-stretching overtone spectra have been used to obtain information about simple conformational equilibria in cases where the relative populations of the conformers are appre~iab1e.I~For CpAm, CpMK, and CMCp, one conformer is clearly dominant. Moreover, the relatively broad line widths of the liquid-phase spectra tend to obscure conformational information, particularly about conformers present in very small concentrations. For these reasons, in our analysis of the overtone spectra, we will assume the existence of single conformers for CpAm(trans), CpMK(cis), and CMCp(gauche). The overtone spectra of the ring C H bonds of CpBr, CPCN, CpAm, and CpMK can be analyzed in a manner that differs from the analysis of the corresponding spectra of CMCp. In this paper, we will first discuss the spectra of the former four molecules before turning our attention to the spectra of CMCp and, finally, to some general considerations.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 i
30
5195
i
l2>
MONOSUBSTITUTED
20 W
0 z a m cc 0 cn m
a
I O
00
MONOSUBSTITUTED CYCLOPROPANES
Experimental Section All of the compounds were obtained from the Aldrich Chemical Co. (CpBr (99%), CpCN, CpAm (98%), CpMK (95%), C M C p (98%)). All expect CpMK were used without further purification. CpMK was distilled prior to the measurement of its spectra. The overtone spectra were recorded on a Beckman 5270 spectrophotometer with the near-IR light source in the regions of Av = 2, 3, and 4, and the visible light source for the regions of Av = 5 and 6. The overtone spectra of the pure liquids in the regions corresponding to Au = 2 and 3 were measured with cells of path lengths 0.1 and 1 cm, respectively. The overtone spectra of the pure liquids in the regions corresponding to Av = 4-6 were measured with 5 cm path length cells. The overtone spectra of C M C p were also measured in the gas phase in the regions corresponding to AucH = 2-4. The spectra were measured with a Wilks variable path length gas cell (Wilks Scientific Corp., South Norwalk, CT, Model 5720) with KBr windows. The cell path length and vapor pressures were varied to obtain optimally measurable signals for the various overtones (see Figure captions). All of the spectra in a digital format were transferred to a Nicolet 1280 computer and converted to a linear energy scale. The spectra were plotted in wavenumber units with a Nicolet Zeta 160 plotter. In the region of Av = 6, the spectra exhibited a (27) Hendricksen, D. K.; Harmony, M . D. J . Chem. Phys. 1969, 51, 700. (28) Pelissier, M.; Leibovici, C.; Labarre, J. F. Tetrahedron 1972, 28, 4825. (29) Mochel, A. R.; Boggs, J. E.; Skancke, P. N. J . Mol. Struct. 1973,15, 93. (30) Kalasinsky, V. F.; Powers, D. E.; Harris, W. C. J . Phys. Chem. 1979, 83, 506. (31) Bartell, L. S.; Guillory, J. P.; Parks, A. T. J . Phys. Chem. 1965, 69, 3043. (32) Lee, P. L.; Schwendeman, R. H. J . Mol. Spectrosc. 1972, 4 1 , 84. (33) Powell, D. L.; Klaboe, P.; Christensen, D. H. J . Mol. Struct. 1973, IS, 77. (34) Pierre, J . L. Ann. Chim. (Paris) 1966, I , 383. (35) Kalasinsky, V . F.; Wurrey, C. J. J . Raman Spectrosc. 1980, 9 , 315.
I/
(cm-')
Figure 2. Liquid-phase overtone spectra of monosubstituted cyclopropanes in the region of AUCH = 3. Spectra were measured at room temperature with a path length of 1.0 cm. Absorbances of cyclopropylamine, cyclopropyl cyanide, and cyclopropyl bromide have been offset by 0.6, 1.2, and 1.8 absorbance units, respectively.
slightly sloping base line. Base-line corrections for these spectra were carried out in the Nicolet 1280 computer prior to plotting. All of the overtone spectra were decomposed with a Fortran 77 ~ fitted Lorentzian peaks curve analysis program, N I R C A P , ~which (36) The Fortran 77 program, NIRCAP, was written by R. K.Marat and modified by A . W. Tarr. ( 3 7 ) Patel, C. K. N.; Tam, A . C.; Kerl, R. J. J . Chem. Phys. 1979, 71, 1470.
5196
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Ahmed and Henrv
TABLE I: Observed and Calculated CH Stretching Peak Positions (cm-') for the Ring CH Bonds of Monosubstituted Cvcloorooanes
C,H,Br
C,HSCN
obsd
calcd
obsd
5 890 5 947 6 000 6 I27 8 749 8 749 8814 8 897 9 102 1 I433 1 1 433 I 1 506
5 879 5 946 5 993 6 I27 8 694 8 722 8816 8919 9111 I 1 395 I I 402 1 1 504 1 1 667 11810 13973 I 3 974 14116 I 4 362(+) I4 44 1 (-) 16 433 16593 16965(+) 16993(-)
5 933 5 993 6 026 6 164 8 799 8 799 8 852 8 962 9 I57 1 1 498 I 1 498 I 1 571
I I827 14 006 14 006 I 4 100 14 505 I6 469 16624 17073
I
,
c
5 934 5 989 6 022 6 I64 8 765 8 785 8 857 8 985 9 I63 1 I473 I I477 1 1 574 1I762 I 1 885 14054 14055 14 I73 14474(+) 14 535(-) 16515 16 654 17080(+) 17 099(-)
1 1 893 14 078 14 078 14 171
14576 I6546 16661 I7 173
CMONOSUBSTITUTED I
CYCLOPROPANES
C3HsNH2
calcd
C3HSCOCH3
obsd
calcd
obsd
calcd
5 895 5 947 5 976 6 I23 8 734 8 734 8 785 8 938 9 092 11 421 11421 I 1 508
5 885 5 945 5 976 6 I23 8 695 8 718 8791 8918 9 102 I I383 1 1 388 1 1 490 11 669 1 1 799 13 945 13 946 14074 14 358(+) 14 426(-) 16387 16542 16946(+) 16 967(-)
5 897 5 959 5 995 6 141 8 758 8 758 8 822
5 892 5 958 5 995 6 141 8710 8 737 8818 8 937 9 131 11 410 11 417 1 1 526 11 689 1 1 831 13 986 13987 14 1 I7 14 384(+) 14 463(-) 16441 16592 16 985(+) 17012(-)
1 1 802 I3 972 I3 972 14 066
14 494 16416 16539 17 096
9 122 11 451 11451 I 1 516 11 820 14019 14019 I 4 I24
14 521 16471 16 592 17 104
assignment
I
I
MONOSUBSTITUTED CYCLOPROPANES
=4
006
z a m L2L
0
m
$
004
0 00 14500
V (cm-l) Figure 3. Liquid-phase overtone spectra of monosubstituted cyclopropanes in the region of AccH = 4. Spectra were measured at room temperature with a path length of 5.0 cm. Absorbances of cyclopropylamine, cyclopropyl cyanide, and cyclopropyl bromide have been offset by 0.2. 0.4, and 0.6 absorbance units, respectively. to the experimental bands. The experimental and fitted spectra were plotted and compared to check the quality of the deconvolution fit. In general, Lorentzian peaks resulted in very good fits to the experimental spectra. However, for the liquid-phase spectra, there was a small indication of Gaussian character in the wings of the bands. Results and Discussion Spectral Analysis of the Ring CH Bonds. The liquid-phase overtone spectra of CpBr, CpCN, CpAm, and CpMK in the regions of AuCH = 2-6 are shown in Figures 1-5. The observed peak positions associated with the ring C H bonds are listed in Table I. These positions correspond to the maxima of the in-
14000
I3500
Liquid-phase overtone spectra of monosubstituted cyclopropanes in the region of AuCH = 5. Spectra were measured at room temperature with a path length of 5.0 cm. Absorbances of cyclopropylamine, cyclopropyl cyanide, and cyclopropyl bromide have been offset by 0.02, 0.04, and 0.06 absorbance units, respectively. Figure 4.
dividual Lorentzian components which were obtained from curve decomposition. When the individual peaks are resolved, the peak positions are known quite accurately to 55 cm-I. However, when the peaks overlap, the uncertainties increase, especially for lowintensity peaks in the region of high-intensity peaks. In these circumstances, the uncertainty can be as high as 10 cm-I. The cyclopropyl ring of monosubstituted cyclopropanes has two types of C H oscillators (methylene and methine). We will assign these spectra by assuming that the stretching vibrations of the CH oscillators on different carbon atoms are uncoupled. Under this assumption, the symmetrized vibrational states associated with the methylene CH oscillators are25
Spectra of Some Monosubstituted Cyclopropanes I I 1 MONOSUBSTITUTED CYCLOPROPANES
0OIOC
/\
-Or
W 0
z a
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5197 TABLE 11: Local-Mode Parameters (cm-') for the Nonequivalent Oscillators of Monosubstituted Cyclopropanes and the Cycloalkanes and Cycloalkenes oscillator 0 wx y'w molecule type 3144.6 f 0.7 57.2 f 0.2 55.1 cyclopropyl -CH2 3169 f 6 57.7 f 1.5 bromide -CH 3173 f 3 59.5 f 0.6 49.4 cyclopropyl cyanide -CH2 -CH 3188 f 2 58.9 f 0.5 3150 f 2 59.2 f 0.5 51.6 cyclopropylamine -CH2 -CH 3162 f 4 57.8 f 1.0 -NH
m
3514 f 4 3154 f 1 3171 f 3 3032 f 6 -CH,(OP) 3087 f 11 -CH,(ip) ring CH" 3159 f 2 ring CHb 3165.7 f 0.3 chlorometh yl" 3082 f 5 chloromethylb 3076 f 5 3061 f 7 equatl axial 3049 f 5 3009 f 10 equatl axial 2987 f 12 3003 f 25 -CH2 3027 f 25 -CH2 3057 f 9 -CH2 C=CH 3163 f 7 2996 f 4 -CH2 3111 f 4 C=CH 3016 f 16 -CH, 3121 f 12 C=CH 3148.6 aryl
cyclopropyl methyl -CH, ketone -CH
[r
0
v,
m
a
chloromethylcyclopropane cyclopentaned I
J P
I
cyclohexaned cycloheptaned cyclooctaned cyclopentaned
-
z/ (cm-0 Figure 5. Liquid-phase overtone spectra of monosubstituted cyclopropanes in the region of AucH = 6. Spectra were measured at room temperature with a path length of 5.0 cm. All spectra are base-line corrected. Absorbances of cyclopropylamine, cyclopropyl cyanide, and cyclopropyl bromide have been offset by 0.002,0.004, and 0.006 absorbance units, respectively.
where Iul,02) = l u 1 ) l u 2 ) is the Morse oscillator product state corresponding to the two C H bonds. The CH-stretching vibrational states associated with the unique methine C H bond can be represented by single Morse oscillator states lu). The methine C H oscillator gives rise to a single peak a t each overtone level, and the energies of these peaks can be obtained simply from the vibrational energy expression given in eq 1. The energies of the methylene C H states can be obtained through an application of the harmonically coupled local-mode model for an XH, system.25 The Hamiltonian is written in the form
H = Eo
cyclohexened cycloheptened benzene'
81.3 f 0.9 58.4 f 0.3 58.0 f 0.6 56.4 f 1.3 57.9 f 2.4 60.5 f 0.4 59.4 f 0.1 61.7 f 1.2 57.9 f 1.5 63.1 f 1.3 67.2 f 0.9 60.4 f 1.9 62.0 f 2.2 62.0 f 4.1 66.0 f 4.1 67.3 f 2 59.4 f 1.6 60.1 f 0.9 59.2 f 0.9 63.9 f 3.5 62.8 f 3.0 57.6
31.9 55.2
43.8 41.8 16.0' 16.0
"Liquid phase. bGas phase. 'Assumed value, see text. dFrom ref e From ref 37.
15.
1
CYCLOPROPYL CYANIDE 0
Ch CH2
3000,
2900
1
+ ( u I + U ~ ) W- (uI2 + ~ 2 +, ~1 + U , ) W X + (al+
- a1)(a2+ - a 2 h w + (a1+ + a1)(a2+ + a2Mw (3)
In eq 3, u1 and u2 are the quantum numbers of the two C H oscillators, Eo is the wavenumber energy of the ground state, and y and 4 characterize the off-diagonal kinetic and potential energy coupling, respectively. These parameters are defined in terms of Wilson G and F matrix elements (y = -'/2(G12/Gll), 4 = 1 / 2 (Fl2/FIl)). The operators al+,a l , etc. have step-up, step-down properties and cause coupling between the two methylenic C H bonds. The details of this model have been described in our previous work on the overtone spectra of the d i h a l o m e t h a n e ~ .The ~ ~ model accounts for all of the anharmonicity that is diagonal in a single C H oscillator. However, coupling between the simple product basis states is restricted to a given vibrational manifold and to the harmonic limit; Le., states IuI,u2)couple only with states l u , f l , ~ ~ 7 1 A) calculation . of the energies for transitions to the states lo), lu,O),, 1 ~ 1 ,,) l etc., requires (i) the anharmonicity constant wx and the harmonic frequency w for the methylene and methine C H bonds; (ii) the effective coupling parameter y'w (y' = y 4), which appears in the intramanifold coupling matrices25of eq 3 for the methylene C H bonds. The parameters w and wx for the methylene and methine C H bonds are evaluated by fitting the observed energies of the Iu,O), and 10) peaks, respectively, to eq 1 with a least mean squares analysis. These values are given in Table 11. The quality of the
4
2600l
2700
1
0
I
1
2
4
6
v Figure 6. Plots of the vibrational energy equation of a single Morse oscillator for the methylene and methine CH oscillators of cyclopropyl cyanide. fit, and the validity of the analysis, are illustrated in Figure 6 which displays a plot of eq 1 for the overtone spectra of CpCN. The effective interoscillator coupling parameter y'w can be calculated from either of the following equations:
E(12,0)+) = 2w - ~ E(I1,l)) = 2w
- [ ( w x ) ~+ ( 2 y ' ~ ) ~ ] ' / *
(4)
+ (2y'"]''2
(5)
W X
- 5wx + [ ( w x ) 2
Equations 4 and 5 are obtained by diagonalization of the 2 X 2 matrix of the Hamiltonian of the XH2 system over the harmonically coupled 12,0)+ and 11,l) states.25 In this work, we have used eq 5 in obtaining y'w since the local-mode peaks 12,0)+are located in the near vicinity of combination peaks marked C in Figure I ,
5198 The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Ahmed and Hem) IO000
CYCLOPROPYL CYANIDE
9800
9600
9400
075
AvcH = 3
DECONVOLUTED SPECTRUM 1
3
4
W
9200
9oco -
8800
860C
1/ (cm-’)
Figure 7. Upper trace: liquid-phase overtone spectrum of cyclopropyl cyanide in the region of .lacH = 3. The spectrum was measured at room temperature with a path length of 1.0 cm. Middle trace: individual Lorentzian functions fit to the experimental spectrum Lower trace: difference of the experimental and fit spectra.
and may be under some perturbation (vide infra). The values of y‘w are listed in Table 11. Substitution of the local-mode parameters of Table I1 into the intramanifold coupling matrices25 of the Hamiltonian (eq 3) followed by diagonalization of these matrices gives the calculated energies of the peaks corresponding to the methylene C H bonds. As we have noted, the energies of the peaks associated with the methine C H bonds can be obtained straightforwardly from eq 1. The calculated and observed energies of the purely CH-stretching peaks associated with the ring CH bonds of CpBr, CpCN, CpAm. and CpMK are given in Table I, along with the peak assignments. The agreement between calculated and observed peak energies is reasonably good, particularly for the dominant peaks, Ic,O), and IC). The observed positions of the weaker Iz1-1,1), peaks are known much less accurately. The local-mode peak assignments for CpBr are indicated in Figures 1-5. The assignments for the local-mode peaks of the ring CH bonds of CpCN, CpAm, and C p M K are analogous although the features are often not as well resolved in the latter three molecules. The discussion in the following paragraphs applies to the spectra of all four cyclopropanes. In Figure I , the splitting of the 12,0)+and 12,O)- peaks is evident, as it is in the dihalomethanes25 and the deuteriated dihalome thane^.^' This splitting arises from the harmonic coupling of the 12,0)+and 11,l) states. The 12) peak is resolved only in CpBr. In CpCN and CpMK, this peak appears as an unresolved shoulder on the higher energy side of the 12,O)- peak. In CpAm, the 12) and 12,O)- peaks are overlapped and give rise to a single broad peak. At hCH = 3 (Figure 2), the 13,0)+ peak is not resolved from the 13,O)- peak. The symmetry splitting is much smaller because of the absence of first-order coupling.25 Because the 13,0)+ peak is buried on the low-energy side of the 13,O)- peak, the agreement between the “observed” and calculated values for 13,0)+ is consistently worse than for 13,0)_. Transitions to the 12,I ) + and 12,l)states are clearly well resolved in the spectra of CpBr and CpCN. This splitting is comparatively larger than the 13,0)+, 13,O)splitting because the product states 12,l) and 11,2) undergo first-order coupling 2 5 However, 12,1)+ is not resolved in either CpAm or CpMK because of overlapping with a combination peak (vide infra) a t higher energy, and the broad 13),13,0), peak a t lower energy. As is the case at l o C H = 2, the methine CH peak, (38) Ahmed, M. K.; Henry, B. R. J . Phys. Chenz. 1986. 90. 10x1.
0 00
I
6900
6600
-
6300
I/ (cm-l)
Figure 8. Liquid-phase room temperature overtone spectra of cyclopropylamine in the regions of A+, = 2 (lower spectrum) and AaYH= 3 (upper spectrum). Path lengths for AuyH = 2 and AaNH= 3 were 0.1 and 1.0 cm, respectively. Lower abscissa and left-hand side ordinate refer to AcNH= 2 spectrum; upper abscissa and right-hand side ordinate refer to ha,, = 3 spectrum. TABLE 111: Observed and Calculated NH-Stretching Peak Positions (em-’) for the NH Bonds of Cyclopropylamine .la,, obsd calcd assignment
2 3
4
5 6
6 501 6 543 6 724 9 558 9 558 9 849 9 934 12435 12873
15 115 17 680
6 517 6 540 6 724 9 554 9 558 9 838 9 962 12 420 12868(+) 12 924(-) 15 121 17 659
12,0)+ I2,O)11,1)
13,0)+ 13,0)-
12,1)+ l2,l)14,o~
13,1)+ 15,0)+ 16,0)+
13), is only resolved for CpBr. For the other three molecules, a single asymmetric peak is observed due to overlapping of transitions to 13) and 13,0)+. However decomposition of these asymmetric peaks clearly shows the presence of two major components. An example of our deconvolution procedure is presented in Figure 7 for the hCH = 3 spectrum of CpCN. At AvCH = 4 (Figure 3), the methine C H peak, 14), is not resolved for all four molecules. For CpBr, this peak appears as an unresolved shoulder to the high-energy side of the more intense 14,0)+ peak. For the other three molecules, the 14,0), and 14) peaks form a single asymmetric peak. For both SoCH = 5 and 6 (Figures 4 and 5), the spectra are dominated by a single asymmetric peak corresponding to transitions to 15,0)+ and I S ) , and to 16,0), and 16), respectively. Further support for our analysis can be obtained by comparing the spectra of the monosubstituted cyclopropanes to the photoacoustically measured gas-phase spectrum of cyclopropane at AccH = 6.16 The hcH = 6 spectrum of cyclopropane is a single symmetric peak. For the monosubstituted cyclopropanes, the overtones have an additional high-frequency peak, or asymmetry on the high-frequency side of the dominant peak. Such additional contributions would be expected since the electron-withdrawing nature of the substituents In these molecules (Br, CN, NH2, and CH3CO) increases the strength and vibrational frequency of the
Spectra of Some Monosubstituted Cyclopropanes
T I 15500
15000
o 015
CYCLOPROPY LAMlNE
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5199 TABLE IV: Observed CH-Stretching Peak Positions (cm-I) for the Methyl CH Bonds of Cyclopropyl Methyl Ketone ADCH obsd position assignment” 2 3
4 5 6
I
\
I
a
W
0
Z
-
d
0 10
m LT
0
cn m
t
0 05
a
0 00
Figure 9. Liquid-phase overtone spectra of cyclopropylamine in the regions of A u N H = 4 (lower spectrum) and A U N H = 5 (upper spectrum). Spectra were measured at room temperature with a path length of 5.0 cm. Lower abscissa and left-hand side ordinate refer to AuNH = 4 spectrum; upper abscissa and right-hand side ordinate refer to A u N H = 5 spectrum.
0 0000
c I8000
17753
-
I7530
1/ icm-’1
Figure 10. Liquid-phase overtone spectrum of cyclopropylamine in the region of A u H N = 6. Spectrum is base-line corrected and was measured at room temperature with a path length of 5.0 cm.
methine C H oscillator relative to the methylene C H oscillators. Spectral Analysis of the NH Bonds of CpAm. The liquid-phase NH-stretching overtone spectra of CpAm in the regions of AvNH = 2-6 are shown in Figure 8-10. The observed peak positions, which were obtained from curve decomposition, are listed in Table 111. The observed peaks of Figures 8-10 can be understood as arising from transitions to symmetrized states” Iu1,u2)*, where VI = AuNH. The energies of these peaks can be calculated in exactly the same fashion that was used to calculate the energies of the peaks corresponding to the methylene ring C H oscillators. The values of the three local-mode parameters, w , w x , and y’w, are given in Table 11. Once again the value of y’w was obtained on the basis of the energy of the 11,l) peak and eq 5. With these parameters, the energies of the NH-stretching peaks for A u N H = 2-6 were calculated and are compared to the observed peak energies in Table 111 along with the peak assignments. The agreement between the
+
5 729 5810 8 404 8 603 11 020 11 198 13464 13702 15 818 16067
For explanation of notation, see text
calculated and observed peak energies is very good. The coupling parameter y’w is much smaller for the N H bonds of CpAm than for the methylene bonds of the cyclopropane ring (see Table 11). This weaker coupling shows up clearly in the overtone spectrum at AvNH = 2 (see Figure 8). Transitions to the 12,0)+ and 12,O)- states are not resolved because of the smaller coupling between 12,0)+and 11,l). The decrease in this coupling is also responsible for the much lower relative intensity of the 11,l) peak.43 At AvNH = 3 in Figure 8, the spectrum is dominated by a single peak, 13,0)*; however the very low intensity peaks, 12,1)+ and 12,1)-, are just discernible on the high-energy side of the main peak. The A u N H = 4, 5, and 6 spectra (Figures 9 and 10) consist essentially of a single peak. However, the A u h H = 5 and A u N H = 6 spectra show some asymmetry about 84 and 138 cm-I, respectively, on the high-energy side of the band maxima. The most probable origin of this asymmetry is an unresolved contribution of the combination involving uNH - 1 local quanta and two quanta of the methine C H in-plane bending mode u7 (1371 CIT-I).~O Spectral Assignment of the Methyl CH Bonds of CpMK. The observed peak positions associated with the methyl C H oscillators of CpMK (Figures 1-5) are listed in Table IV along with their assignments. These positions correspond to the maxima of the individual Lorentzian components from curve decomposition. The methyl C H peaks of CpMK are analogous to those found for acetone. Fang and Swofford” and Fang et aLI9 have analyzed the photoacoustically detected AuCH = 4-7 spectra of acetone. Hanazaki et al.39have assigned the gas-phase spectra of acetone in the region of AuCH = 1-4. This work followed earlier work by M ~ K e a n ~ who ~ . ~ studied ’ the fundamental CH-stretching spectrum of CD3COCD,H. All of these studies have shown that a methyl group in acetone has two conformationally nonequivalent C H bonds. One C H bond, CH,, lies in the molecular plane that includes the carbonyl group, while the other two C H bonds, CH,, lie out of this plane. The spectra17,19139-41 show two well-resolved peaks, a higher frequency peak corresponding to the stronger CH, bond, and a lower frequency peak corresponding to the weaker CH, bonds. We assign the methyl C H peaks by assuming that the stretching vibrations of the CH, oscillators are essentially uncoupled from the stretching vibrations of the CH, oscillators. Such an approximation has proved to be successful as a basis for the analysis of the overtone spectra of 2-chloro-2-methylpropane and chlor ~ t r i m e t h y l s i l a n e . ~Thus, ~ the symmetrized local-mode states associated with the methyl C H bonds of CpMK can be written as ( U , , D ~ ) + , ~ U ~ )where ~, ul u2 + u3 = AuCH for the methyl C H bonds. For example, at A u c H = 2, the following states can occur: 12,0)*, ~ ~ ~ 2 , 0 ~11,1 ~ ) ,a (=11,1 ~ 0 ~),IO),), , ~ , 12), (=10,0)a12)s),and l1,O)*al1 ),* Only two types of methyl C H peaks are observed in the overtone spectra of CpMK. These peaks correspond to transitions to states Iu,O)+, or I U ) ~ , Le., pure local-mode states whose components have
+
(39) Hanazaki, I., Baba, M.; Nagashima, U. J . Phys. Chem. 1985, 89, 5637. (40) McKean, D. C. Chem. Commun. 1961, 1373. (41) McKean, D. C. Spectrochim. Acta, Part A 1975, 31A, 861. (42) Ahmed, M. K.; Henry, B. R. J. Phys. Chem. 1987, 91, 3741
5200
TABLE V: Positions (cm-') and Tentative Assignments for the Combination Peaks Observed in the Overtone Spectra of Monosubstituted Cyclopropanes AcCH
2
molecule C,H5Br
obsd position 5 840 5 860
3
8 648
8 990 2
C3H5CN
5 733
5 900
8 671
3
9 044
2
C3H5NH2
5 688
5 771 5 848 6 301 3
8 325
8 463 8 982 A
1 1 040
10772 2 3
Ahmed and Henrj
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
C,HSCOCH,
5616 5 771 8 504 9 006
TABLE VI: Observed and Calculated CH-Stretching Peak Positions (cm-') for the Ring CH Bonds of ChloromethvlcvcloDroDane ~
liquid
tentative assignmentQ
sym C H , str v I (3012 cm-I) + 2 X u I 5 asym C H , def (1422 cm-I) sym CH, str v I (3012 cm-l) + 2 X u4 sym CH, def (1446 cm-I) 12,0)+ (5890 cm-I) + 2 X u l S asym CH, def (1422 cm?) 11.1) (6127 cm-I) + 2 X u4 sym CH, def ( 1 446 cm-l) deg CH, str vl (3058 cm-') + 2 X u6 C H def ( I 348 cm-I) sym CH, str v, (3030 cm-I) + 2 X u5 sym CH, def (1460 cm-I) 12.0)- (5993 cm-I) + 2 x v6 sym C H def ( 1 348 cm-I) 1 1 , l ) (6164 cm-I) + 2 X v 5 sym C H 2 def (1460 cm-I) C H str u j (2964 cm-I) + 2 X v, C H bend in plane (1371 cm-') antisym stretch u, (3084 cm-') + 2 X u , C H bend in plane (1371 cm-I) antisym stretch u , (3084 cm-I) + 2 u 1 9 sym CH, def (1418 cm-I) antisym N H stretch u16 (3368 cm-I) + C H stretch v j (2964 cm-') 11.1) (6123 cm-l) + 2 X v20 CH, twist (1101 cm-l) 11.1) (6123 cm-I) + 2 X u8 ring breathing (1 21 3 cm-l) 11.1) (6123 cm-') + 2 X v6 CH, def (1453 cm-I) 13.0)* (8734 cm-I) 2 X uq CH, twist (1168 cm-') )3.0), (8734 cm-I) + 2 X v21 CH, wag (1037 cm-I) 2962 cm-l + 2 X 1352 cm-' 2962 cm-l + 2 X 1420 cm-' 12), (5810 cm-I) + 2 X 1383 cm-' 1 1 , l ) (6141 cm-I) + 2 X 1442 cm-l
L C H
obsd
calcd
obsd
calcd
assignment
2
5911 5958 6122 8747 8747 8924 9094 11426 1 1 426
5909 5955 6122 8720 8734 8936 9097 11403 1 I406 1 1 704 11809 12017 13960 14 399 14445 16394 16978 16989
5930 5975 6137 8785 8785 8961 9118 11475 1 1 475
5932 5975 6137 8756 8769 8967 9121 11454 I 1 456 11751 11850 12050 14028 14462 14504 16482 17056 17066
12.0)+ )2,0)_,12) 11,l) 13,0)+ 13,0)_,13) JL,I)+ 12,l)14,0)+ 14,0)_, 14) /3,1)+ 13,l)12,2) 15,0),, 15) /4.l)+ 14,l)16.0)*, 16) 15,1)+ 15,l)-
3
4
11785
all of the vibrational quanta deposited in one of the three methyl C H bonds. Generally, local-mode combination peaks where the vibrational quanta are distributed over at least two C H bonds are observed a t low overtones. However, for the methyl C H bonds of CpMK, these peaks are absent even a t AvCH = 2. Most of the expected local-mode combinafion peaks are also absent in the AucH = 2 and 3 spectra of acetone and a t I v C H > 3 such peaks are not It has been shown that the dominant source observed a t a11.17J9339 of intensity for local-mode combination states is vibrational mixing of these states with pure local-mode states.43 Thus the absence of such combination states in the spectra of acetone and CpMK supports the assumption of decoupled CH, and CH, oscillators and suggests only weak coupling between the two CH, oscillators. Once again the energies of the peaks associated with the CH, oscillators could be obtained through an application of the harmonically coupled local-mode model for an XH, system,25while the CH, energies could be obtained straightforwardly from eq I . Such an analysis has been used for the spectra of 2-chloro-2methylpropane and chl~rotrimethylsilane~~ and a related procedure has been used for the spectra of acetone.39 However, in this work, because these peaks are very weak and their positions subject to a large uncertaint), we have not carried out such a calculation but have ?imply identified the nature of the methyl peaks.
13978
6
14484 16421
11833
TABLE VII: Observed and Calculated CH-Stretching Peak Positions (cm-') for the Chloromethyl CH Bonds of Chloromethvlcyclopro~ane liquid calcd
AcpM
obsd
2
5788 5788 5911 8514 8514
3
4 5 6
11 114 13553 15896
5787 5795 5926 8503 8504 8 725 8 789 11093 13559 15901
gas obsd
calcd
assignment
5803 5803 5930 8539 8539
5796 5805 5929 8529 8530 8737 8800 1 1 143 13640 16021
12,0)+ 12,O)11,l) 13,0)+ 13,O)12,1)+ 12,l)14,0), 15,0), 16,0),
1 1 143
The intensity of the methyl peaks of CpMK is relatively low compared with the intensity of the peaks associated with either the ring CH bonds of all of the cyclopropyl molecules or the N H > 3), the relative bonds of CpAm. For the higher overtones (IC peak intensity per XH bond follows the order: N H > methine ring CH > methylene ring C H > methyl C H . Clearly electronic structure plays an important role in determining overtone intensi ties. Combination Peaks. The peaks marked C in Figures 1-3 are combination peaks, and appear in the AucH = 2 and AccF, = 3 spectral regions of all four molecules, and also in the ItCH =4 spectral region of CpAm. These peaks are relatively low in intensity. With one exception, the peak a t 6301 cm-' in CpAm, they arise from transitions to states with u - 1 quanta in C H stretching and two quanta in a lower frequency mode, usually one that involves CH2 deformation. Such peaks are commonly observed in the overtone spectra of several molecule^.'^ 14.25.44-49 The observed peak positions, which were obtained from curve decomposition, are listed in Table V along with tentative assignments. The assignments of these combination peaks are based on frequencies obtained from studies of the fundamental spectra for CpBr,SoC P C N , ~CpAm,l0 ~ and C P M K . ~ ~ Often these combination peaks interact with the pure C H stretching peaks. Such interactions can cause shifts in the peak positions which contribute to the differences between the calculated ~~
~~~
~
~~~
(44) Fang, H. L.; Swofford, R. L. J. Chem. Phys. 1980, 72, 6382. (45) Fang, H. L.; Swofford, R. L . J. Chem. Phys. 1980, 73, 2607. (46) Henry, B. R.; Mohammadi, M. A.; Hanazaki, I.; Nakagaki, R. J. Phys. Chem. 1983, 87, 4821. (47) Voth, G. A,; Marcus, R. A,; Zewail, A. H. J . Chem. Phys. 1984, 81, 5494. (48) Peyerimhoff, S.; Lewerenz, M.; Quack, M. Chem. Phys. Lett. 1984. 109. 563. ~
(43) Mortensen, 0 .S.: Ahmed, M.K. Henry. B. R.; Tarr. A. W. J . Chem. Phys. t385, 82, 3903.
5
17100
+
Fundamental frequencies for C3H5Br. C,H5CN, C,H5NH,, and C,H,COCH, are from ref 50. 51 30. and 33. respectively.
~~~
gas
1~~~
(49) Perry, J. W.; Moll, D. J.; Kuppermann, A,: Zewail, A . H. J. Chem. Phys. 1985, 82, 1195. (50) Rothschild, W. G. J. Chem. Phys. 1966, 44, 3875. (51) Daly, L. H.; Wiberley, S. E. J . Mol. Specfrosc 1958, 2, 177.
Spectra of Some Monosubstituted Cyclopropanes
Figure 11. Lower trace: liquid-phase overtone spectrum of chloromethylcyclopropane in the region of AVCH= 2. The spectrum was measured at room temperature with a path length of 0.1 cm. Upper trace: gas-phase overtone spectrum of chloromethylcyclopropane at 90 O C in the region of AuCH = 2. Path length, 2.25 m.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5201
Figure 13. Lower trace: liquid-phase overtone spectrum of chloromethylcyclopropane in the region of AUCH = 4. The spectrum was measured at room temperature with a path length of 5.0 cm. Upper trace: gas-phase overtone spectrum of chloromethylcyclopropane at 90 O C in the region of AuCH = 4. Path length, 12.75 m.
A: AVCH
I4500
14000
5
13500
i7 (cm-I)
Figure 14. Liquid-phase overtone spectrum of chloromethylcyclopropane in the region of AvCH = 5. The spectrum was measured at room temperature with a path length of 5.0 cm.
Figure 12. Lower trace: liquid-phase overtone spectrum of chloromethylcyclopropane in the region of AvCH = 3. The spectrum was measured at room temperature with a path length of 1.0 cm. Upper trace: gas-phase overtone spectrum of chloromethylcyclopropane at 90 O C in the region of AuCH = 3. Path length, 8.25 m.
and observed frequencies in Table I. For example these differences are greater for 12,0)+ than for 12,0), and for 12,1)+ than for 12,1)-. In each case there is a combination state in close proximity to the symmetric state. Chloromethylcyclopropane. The liquid- and gas-phase overtone spectra of C M C p in the regions of AucH = 2-4 are shown in Figures 11-1 3. The liquid-phase overtone spectrum in the region of A u ~ H= 5 is shown in Figure 14. The liquid-phase overtone spectrum in the region of AucH = 6 was also measured but is not displayed. The spectral features of this latter spectrum were similar to the spectral features of the overtone spectra in the regions of AocH = 4 and 5 . The observed peak positions, which were obtained from curve decomposition, are listed in Tables VI and VII.
The spectral features which correspond to the ring C H oscillators of CMCp differ in one important respect from the analogous features in the spectra of CpBr, CpCN, CpAm, and CpMK. Peaks corresponding to the methine C H oscillator are not resolved a t any overtone. For the higher overtones (AucH = 4-6), the ring C H oscillators give rise to a single symmetric peak a t each overtone. This lack of resolution of the two oscillator types is an indication that the bond strengths/lengths of the methylene and methine C H bonds of C M C p are nearly the same. Such a similarity is not unexpected. Methyl groups tend to be electron donating and the chloro substituent is electron attracting, so the two effects will work in opposing directions with regard to perturbation of the cyclopropane ring. Once again, we assign the spectra associated with the ring C H bonds by assuming that the stretching vibrations on the different carbon atoms are uncoupled. The energies of the states for both the ring and chloromethyl CH oscillators can then be obtained through an application of the harmonically coupled local-mode model for an XH2 system.25 On the basis of the suspected equality of the methylene and methine C H bond strengths, the methine CH oscillator peaks are assumed to be coincident with the ring methylene peaks, lu,O)_. The values of the local-mode parameters, w and w x , for the ring methylene and chloromethyl C H oscillators were determined, as before, by fitting the energies of the Ic,O), peaks to eq 1. For the ring methylene CH oscillators, y’w was determined separately
5202
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
for the liquid- and gas-phase spectra on the basis of the energy of the )1,1) peak and eq 5 . However, for the chloromethyl C H oscillators, y'w was determined for both the liquid- and gas-phase spectra from the observed splitting between the gas-phase fundamentals I l , O ) _ (2994 cm-') and l l , O ) + (2963 cm-') (2y'w =
.w,o)-)
-
~(11,0)+)).35
All of these local-mode parameters are listed in Table 11. With these parameters, the energies of the peaks were calculated and are compared to the observed peak energies for the ring methyl C H bonds in Table V I and for the chloromethyl C H bonds in Table VII. The agreement between the calculated and observed peak energies is very good, particularly for the chloromethyl C H oscillators. It is of interest to note that the coupling between the two chloromethyl bonds is much weaker than the corresponding coupling between the ring methylene C H bonds. Note the peaks 12,0)+ for the ring C H bonds and I1,l) for the chloromethyl C H bonds are predicted to be nearly coincident. Thus we have assigned the unresolved shoulder at 591 1 cm-' (liquid) and 5930 cm-' (gas) to both of these transitions (see Tables VI and VII). Note also that the relative intensity of the chloromethyl C H oscillator peaks is much higher than for the methyl peaks of CpMK. In addition to the peaks listed in Tables VI and VII, low-intensity local mode-normal mode combination peaks are also observed at AucH = 2 and 3 in both the liquid- and gas-phase spectra of C M C p (5864, 8583, 8665, and 8987 cm-', liquid; 5877, 8614, 8712, and 9015 cm-', gas). Assignments for these peaks could be made on the basis of the local-mode peak positions (Tables VI and VII) and the frequencies of the fundamental^.^^ Local-Mode Parameters. The errors in w and w x listed in Table I! are derived from a least mean squares analysis. Their relatively small size indicates the excellent quality of the fit to eq 1 for all but the methyl oscillators of CpMK and the chloromethyl oscillators of CMCp. The corresponding peaks for these latter cases are relatively weak, and the peak positions are perturbed by adjacent combination peaks in the regions of AuCH = 2 and 3 (see Figures 1, 2, 11, and 12). In Table 11, we have also listed values of w and wx from previous for a variety of cycloalkanes and cycloalkenes for comparison to the values for the monosubstituted cyclopropanes. The sensitivity of these local-mode parameters to the molecular environment is clearly evident. The local-mode frequencies of the ring C H oscillators of the monosubstituted cyclopropanes are consistently higher than the local-mode frequencies of the methylene C H oscillators of the cycloalkanes and cycloalkenes listed in Table 11. In fact the values of w and wx for the ring C H oscillators of the monosubstituted cyclopropanes are much closer to the corresponding quantities for the olefinic C H oscillators of ~ y c l o p e n t e n e , c' ~y c l ~ h e x e n e , 'and ~ ben~ene.~' The higher frequencies and lower anharmonicities of the methylene ring C H oscillators of the monosubstituted cyclopropanes are primarily the result of ring strain. The high ring
Ahmed and Henrq strain in cyclopropane increases the ''p" character in the endocyclic C-C bonds to maximize the orbital overlap. Consequently the "s" character of the exocyclic C H bond increase^.^^-^^ The increased s character makes them s h ~ r t e and r ~ stronger ~ ~ ~ ~ and their vibrational energy increases. In fact, theoretical models have been suggested with hybridizations of sp2 to sp2.**for the C H bonds of cyclopropane. The data of Table I1 for monosubstituted cyclopropanes are in agreement with such models. In our spectral assignments of CpBr, CpCN, CpAm, and CpMK, we assigned the higher frequency of the two principal components associated with the ring C H bonds to the methine C H oscillators. The local-mode frequencies for the methine C H oscillators are consistently higher than for the methylene C H oscillators. This assignment is also supported by I3C N M R ~ t u d i e s on ~ ~ monosubstituted .~~ cyclopropanes. The coupling , the methine C H oscillators are significantly constants, J C H for higher than the JCH for the methylene C H oscillators. These higher values of JCHcorrespond to more s characters6@' and thus a shorter methine C H bond length,55$56 and a higher methine C H vibrational frequency. Summary
We have successfully analyzed the overtone spectra of several monosubstituted cyclopropanes based on a model which assumes that the stretching motions of the C H bonds located at different carbon centers are uncoupled. Within this description, the various XH2 bonds are treated as harmonically coupled local modes. In CpBr, CpCN, CpAm, and CpMK, our analysis indicates that the methine C H bond is stronger than the methylene C H bonds in the cyclopropyl ring. The bond strength and hybridization of the ring C H bonds in the monosubstituted cyclopropanes are much closer to what is found in olefins than to what is found in larger cycloalkanes.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support. We are also grateful to Professor Ted Schaefer for helpful discussions. B.R.H. is grateful to the Canada Council for a Killam Research Fellowship. Registry No. CpBr, 4333-56-6; CpCN, 5500-21-0; CpAm, 765-30-0; CpMK, 765-43-5; CMCp, 591 1-08-0. (52) Klasinc, L.; Maksic, Z.; Randic, M. J . Chem. SOC.A 1966, 755. (53) Galabov, B.; Morris, H. J . Mol. Struct. 1973, 17, 421. (54) Galabov, B.; Simov, D. J . Mol. Struct. 1972, 11, 341. (55) Muller, N.; Pritchard, D. E. J . Chem. Phys. 1959, 31, 768. (56) Muller, N.; Pritchard, D. E. J . Chem. Phys. 1959, 31, 1471. (57) Bernett, W. A. J . Chem. Educ. 1967, 44, 17. (58) Crecely, K. M.; Watts, V. S.; Goldstein, J. H. J . Mol. Spectrosc. 1969, 30, 184. (59) Weiner, P. H. Malinowski, E. R. J . Phys. Chem. 1967, 71, 2791. (60) Alsenoy, C. V.; Figeys, H. P.; Geerlings, P. Theor. Chim. Acta 1980, 55, 87.
