J. Phys. Chem. 1994,98, 2050-2055
2050
Microwave Spectrum and Structure of MethylcyclopropaneHCl Susan E. Forest, Anne M. Andrews,t and Robert L. Kuczkowski' Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 481 09- 1055 Received: November 2, 1993'
The microwave spectrum of the methylcyclopropane-HC1 (MCP.HC1) complex has been observed using a pulsed nozzle, molecular beam Fourier transform microwave spectrometer. Transitions were assigned with the aid of the quadrupole hyperfine structure from the chlorine nucleus. The rotational constants of the C ~ H T H ~ ~ C ~ speciesareA =6408.1(2) M H z , B = 1410.486(1) M H z , a n d C = 1224.155(1) MHz. Inadditiontothisspecies, the spectra of the MCP-H3'C1 and MCP.D3T1 isotopomers have also been studied. The data are consistent with a structure in which the HCl interacts with the C - C bond of the M C P ring that is adjacent to the methyl-substituted carbon.
Introduction Several complexes of cyclopropane (CPP) have been studied because of its interesting "bent bond" description of the electron density of the carbonxarbon bonds.' These complexes, for example, CPP.HCl,* CPP.HF,3 CPP*HCN,4and CPPsH20,S have shown that cyclopropane accepts hydrogen bonds from other moieties through an interaction between a C-C bond of the cyclopropane ring and the hydrogen of the donor. Truscott and Ault6 used infrared matrix isolation to study complexes of methyl-substituted cyclopropane rings with hydrogen halides. The purpose of that investigation was to determine the interaction site since substitution destroys the equivalence of the carbon atoms, leading to more than one possible site for complexation. Their study showed that methylcyclopropane (MCP) and HC1 interact to form only one species, implying one preferential site for interaction. This was inferred to be the carbonsarbon bond adjacent to the methyl-substituted carbon, hereafter referred to as the asymmetric structure (see Figure la). This result is consistent with the electron-donatinginductive effect of the methyl group, which strengthensthe adjacent carboncarbon bond, making it the preferred site for hydrogen bonding, while simultaneously weakening the C-C bond opposite to the substituted carbon.6 In contrast, ab initio calculations7have found that the preferred structure of the complex of MCPmHF has the H F interacting with the C-C bond that is opposite the methyl-substituted carbon (see Figure lb). This structure will be known throughout the remainder of the paper as the symmetric structure. With this discrepancy between theory and experiment, we have examined the MCP-HCl complex in the gas phase using Fourier transform microwave (FTMW) spectroscopy. FTMW spectroscopy is ideal for this study because the gas-phase technique eliminates any possible interfering interactions that might occur in an infrared matrix study and results in a structure that will allow direct comparison to ab initio calculations. Our conclusion is that the MCP-HCl complex does indeed have the asymmetric form determined by the infrared matrix study of Truscott and Ault.
Experimental Section The rotational spectrum of MCP-HCl was observed using a Balle-Flygare Fourier transform microwave spectrometer equipped with a pulsed nozzle source.* Line widths were typically 15-30 kHz full width at half-maximum with center frequencies estimated to be accurate to approximately f4 kHz. The deuterated species t Current address: Molecular Spectroscopy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. Abstract published in Aduunce ACS Abstracts, February 1, 1994.
(b) Figure 1. Models for MCP-HXcomplex: (a) asymmetric structure from matrix infrared studies; (b) symmetric structure fromabinitiocalculations.
resulted in transitions that were broadened up to 100 kHz or more as the deuterium nuclear quadrupole hyperfine structure was unresolved. These line centers were therefore accurate to 20-30 kHz. A mixture of -2% MCP (Columbia Organic Chemical Co.) with -2% HCl (Aldrich Chemical Co.) and -96% Ar carrier gas, maintained at a total pressure of 1-2 atm, enabled a spectrum to be observed. The normal species and the H W l isotopomer were observed in natural abundance, while the deuterated species was formed with an enriched sample of D3sC1(99% D enrichment, Cambridge Isotope Laboratories). Several days were required to allow conditioning of the sample line and nozzle with the deuterated sample before any DCl transitions were observed.
Results and Analysis SpectralAssignment. The spectrumof MCPmHCl was predicted using models for both the symmetric and asymmetric forms. Transitions were observed and eventually assigned to the asymmetric form (see below) with the aid of the nuclear quadrupole splitting patterns from the chlorine atom. These transitions belonged to an a-type R-branch series that spanned over four Ss.
0022-3654/94/2098-2050!§04.50/00 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2051
Microwave Spectrum of Methylcyclopropane.HC1
TABLE 1: transition
Observed Transition Frequencies for Methylcyclopropane.HjdC1
F"
F'
3.5 2.5 3.5 t.5 4.5 2.5 1.5
3.5 1.5 2.5 0.5 3.5 2.5 1.5
3.5 1.5 4.5 2.5 1.5
3.5 0.5 3.5 2.5 1.5
3.5 2.5 4.5 1.5
2.5 1.5 3.5 0.5
3.5 2.5 3.5 1.5 4.5 2.5 1.5
3.5 1.5 2.5 0.5 3.5 2.5 1.5
4.5 3.5 4.5 2.5 5.5 3.5 2.5
4.5 2.5 3.5 1.5 4.5 3.5 2.5
4.5 3.5 4.5 3.5 2.5
4.5 2.5 3.5 3.5 2.5
4.5 3.5 5.5 2.5
3.5 2.5 4.5 1.5
3.5 4.5 3.5 5.5 4.5 2.5
3.5 3.5 2.5 4.5 4.5 1.5
4.5 2.5
3.5 1.5
V0bU
7 621.069 7 612.460 7 619.057 7 619.188 7 622.343 7 622.495 7 623.883 7 629.102 7 883.288 7 870.789 7 880.626 7 883.976 7 890.085 7 893.723 7 903.498 7 894.078 7 903.499 7 907.258 7 916.692 8 179.937 8 171.611 8 177.984 8 178.039 8 181.287 8 181.344 8 182.591 8 187.733 10 155.530 10 144.957 10 154.290 10 154.992 10 155.605 10 156.315 10 158.983 10 165.652 10 487.306 10 474.522 10 486.170 10 487.707 10 492.340 10 499.353 10 533.847 10 530.487 10 532.346 10 535.772 10 537.606 10 547.312 10 537.646 10 539.256 10 545.331 10 551.121 10 555.749 10 557.208 10 547.839 10 539.782 10 557.734
A@ -1.9 11.9 -3.8 -4.3 -14.6 5.9 4.9 0.1 -48.8 52.7 -24.4 61.1 18.4 107.8 10.5 0.8 1.3 -7.8 5.7 1.4 9.0 -5.1 -7.4 1.1 0.9 -1.2 2.7 0.6 3.2 -2.2 -2.8 -5.5 1.8 5.0 0.4 -1.8 1.3 -5.9 0.5 10.7 -6.6 -5.5 -3.3 6.0 6.5 -9.2 0.1 5.4 -3.0 -2.8 -8.2 4.6 4.0 1.o -3.4 3.4
transition
F"
F'
4.5 3.5 5.5 2.5
3.5 2.5 4.5 1.5
3.5 4.5 4.5 2.5 5.5 3.5 2.5
2.5 3.5 4.5 1.5 4.5 3.5 2.5
5.5 4.5 5.5 3.5 6.5 4.5 3.5
5.5 3.5 4.5 2.5 5.5 4.5 3.5
5.5 4.5 6.5 4.5 3.5
5.5 3.5 5.5 4.5 3.5
5.5 4.5 6.5 3.5
4.5 3.5 5.5 2.5
5.5 4.5 6.5 3.5
4.5 3.5 5.5 2.5
5.5 4.5 5.5 3.5 6.5 4.5 3.5
5.5 3.5 4.5 2.5 5.5 4.5 3.5
5.5 4.5 6.5 7.5 4.5
4.5 3.5 5.5 6.5 4.5
5.5 7.5
4.5 6.5
5.5 4.5 6.5 7.5
4.5 3.5 5.5 6.5
VOb'
10 584.738 10 581.372 10 583.233 10 586.663 10 588.507 10 900.232 10 899.018 10 889.686 10 899.949 10 900.333 10 90l.OOd 10 903.573 10 910.077 12 685.190 12 673.665 12 684.427 12 685.025 12 685.078 12 685.684 12 688.421 12 696.448 13 071.401 13 058.495 13 070.767 13 071.676 13 075.404 13 083.957 13 160.679 13 159.162 13 159.553 13 161.808 13 162.182 13 261.748 13 260.234 13 260.615 13 262.874 13 263.259 13 614.817 13 603.583 13 614.073 13 614.644 13 614.724 13 615.307 13 617.955 13 625.785 15 209.166 15 208.666 15 209.038 15 209.126 15 209.506 15 221.056 15 631.822 15 631.422 15 632.026 16 321.942 16 321.447 16 321.825 16 321.904 16 322.268
Avb 0.5 -8.0 2.8 5.7 -0.5 -0.2 -2.0 -1.9 2.3 -5.2 1.9 7.2 -2.4 1.7 1.3 -2.7 1.9 -10.6 2.0 8.1 0.0 -0.3 -0.7 -7.2 -0.8 7.0 1.7 -2.6 -4.6 4.4 4.1 -3.9 -3.3 -0.4 -1.8 -0.2 2.4 -2.3 -1.7 1.8 -0.1 -5.8 4.3 5.8 -4.2 2.1 1.1 -3.2 0.9 4.7 -3.5 -2.0 -1.9 1.9 4.2 -2.9 -0.6 7.6 -4.1
a Observed frequency (vob) in MHz. The first frequency listed for each transition is the center frequency in the absence of hyperfine splitting. Av = v,b - v,lc in kHz. C This transition was not included in the fit.
The b- and C-type Q-branch series were predicted but unobservable. Dipole moment predictions for the asymmetric structure indicate that pb and peare 5-0.1 D, resulting in weak transitions below the sensitivity of the spectrometer. Nineteen transitions were assigned for the MCP.HSSC1species, followed by 12 and 10 for the MCP-H37Cl, and MCPaD35Cl isotopomers, respectively. The 303-202 transition was omitted from the fitting of all three species because this transition greatly degraded the quality of the fit. This deterioration was the result of a near degeneracy in the 202 and l l 0 levels, resulting in a perturbation which shifts the 303-202 transition 50-100 kHz. The remaining transitions (Table 1) were fit using a Watson S-reduced semirigid rotor Hamiltonian (Prepresentation) to three rotational constants and four distortion
constants. These spectroscopic constants are listed in Table 2. The transition frequencies for the other isotopomers are available as supplementary material. (See the paragraph at the end of the paper regarding supplementary material.) The A rotational constant is not extremely well determined since only a-type lines were observed. Nuclear quadrupole coupling constants were derived for the chlorine nucleus in all three isotopomers ( I = 3/2 for both 35Cl and 37Cl),and these values can be found in Table 2. In addition to the assigned transitions mentioned above, there were approximately 15 regions containingan unassigned transition or transitions with quadrupole-like structure. These transitions
2052
Forest et al.
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
TABLE 2 Spectroscopic Constants for Isotopic Species of the Methylcyclopropane-HC1 Complex MCP.H3'CI A/MHz B/MHz C/MHz Dj/kHz DjK/kHz dl/kHz &/kHz xoo/MH~ Xbb/MHZ
6408.1(2) 1410.486(1) 1224.155(1) 1.392(7) 11.99(6) -0.186(9) -0.028(6) -52.754(5) 25.778(7) na 18 Aum,/kHzb 3.4
MCP*H3'C1
MCP*D3sC1
6406.6(1) 1368.8167(3) 1192.6103(3) 1.309(3) 12.1(2) -0.174(2) -0.028 (assumed)c -41.57(2) 20.27(4) 11 0.6
6403(1) 1409.779( 1) 1223.489(1) 1.35(4) 12(2) -0.186 (assumed)c -0.028 (assumed)c -55.06(6) 26.79(7) 9 4.3
Au = uOb- u,lc. 0 Number of transitions in the fit. in the fit.
Value assumed
were generally weaker than the assigned ones, and they were unable to be fit to either isomer. Structure. Models of the symmetric and asymmetric forms of the MCPeHCl complex were predicted to have very different rotational constants. Using parameters from CPP-HC1and the assumption that MCP and HCl retain their individual structures upon c~mplexation,~JO the symmetric form resulted in A = 12 112.1 MHz,B=988.6MHz,andC=985.2MHz. Compared with the asymmetric form, which had predicted rotational constants of 6730.4, 1433.2, and 1250.2 MHz, respectively, it was apparent that the experimental spectrum was consistent with the latter structure. Furthermore, the small isotope shift due to the substitution of deuterium in the HCl demonstrates that the hydrogen atom is lying between the MCP and the chlorine atom. A much larger change in rotational constants would be expected upon substitution if the hydrogen atom was on the other side of the chlorine atom. However, the deuterium data do not allow for a precise determination of the location of the hydrogen atom. Kraitchman's equations11 were used to determine the Cartesian coordinates of the substituted atoms, and while the values are reasonable for the position of the C1 atom, the position of the hydrogen atom is unreasonable. In fact, the Kraitchman calculations produce coordinates for the chlorine and hydrogen atoms that lead to an anomalous bond length of 1.957 A for the HCl compared to an uncomplexed HCl bond length (RH
