Conformational analysis of (aminomethyl)cyclopropane hydrochloride using vibrational spectroscopy and ab initio calculations

MOLSTR 10349

Journal of Molecular Structure 449 (1998) 159–176

Conformational analysis of (aminomethyl)cyclopropane hydrochloride using vibrational spectroscopy and ab initio calculations Muhammed Ayub a, Chun-Te Ko b, Charles J. Wurrey b,* b

a School of Pharmacy, University of Missouri–Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA Department of Chemistry, University of Missouri–Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA

Received 19 November 1997; accepted 15 December 1997

Abstract Raman spectra (from 100 to 3500 cm −1) of the salt (aminomethyl)cyclopropane hydrochloride and its N-d 3 isotopomer have been recorded in the polycrystalline solid phase and in H 2O and D 2O solutions respectively. In addition, the infrared spectrum of a KBr pellet of the light compound has also been obtained from 4000 to 600 cm −1. Supporting the experimental data, ab initio calculations at the RHF/6-31G* and MP2 levels have been carried out for the cyclopropylmethyl ammonium cation present in both of these molecules. Unlike the structural results obtained from similar studies of the isoelectronic molecules, ethyloxirane and ethylcyclopropane, the cyclopropylmethyl ammonium cation does not exhibit any experimental evidence for the existence of a cis conformer of this species. All of the observed spectral peaks—in both the Raman and infrared spectra—can be assigned as arising from the gauche rotameric form of this ion. This result is comparable with those found for ethyloxirane and ethylcyclopropane, in that the gauche conformer (or conformers, in the case of ethyloxirane) is the predominant form. However, for both of the ethyl-substituted three-membered ring compounds, small amounts of the cis conformer were also detected. We suspect that the positive charge of the cyclopropylmethyl ammonium cation, and the presence of the chloride counter-ion, tend to destabilize the cis structure of the cyclopropylmethyl ammonium ion. The computational results support the structural conclusions from the experimental study. At both the RHF/6-31G* and MP2 levels, the gauche rotamer of the cyclopropylmethyl ammonium ion is predicted to be the more stable form of this species, with the cis conformer also stable, but either 4.8 (6-31G*) or 4.7 kcal mol −1 (MP2) higher in energy and, therefore, not populated at ambient conditions. The Cartesian force constants obtained from the ab initio calculations were transformed into internal coordinate force constants, and a normal coordinate calculation was performed, yielding simulated infrared and Raman spectra of the title compound and its isotopomer. The simulated spectra of the gauche conformer in each case agree very well with those observed experimentally, thereby lending additional credence to our structural assignments. q 1998 Elsevier Science B.V. Keywords: Infrared; Raman; Structure; (Aminomethyl)cyclopropane

1. Introduction It has been shown in several recent papers [1,2] that the combination of infrared and Raman * Corresponding author

spectroscopies with high-level quantum mechanical calculations may be successfully employed to evaluate the conformational behavior of three-membered ring compounds containing an unsymmetrical and saturated substituent such as an ethyl group. For ethyloxirane [1], its fluid phases are comprised

0022-2860/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 02 2- 2 86 0 (9 8 )0 0 45 8 -X

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Table 1 Conformational behavior of some cyclopropyl–CH 2 –X compounds X

Relative abundance of conformers (%) gauche

–Cl –Br –I –CN

Reference

cis

95 98 100 85

5 2 — 15

[3] [4] [5] [6]

predominately of the gauche-1 (major) and gauche-2 conformers, but small amounts of the cis rotameric form were also detected. In the case of ethylcyclopropane [2], the gauche conformer also is the dominant fluid-phase conformer, with the cis conformation also stable, but 1.10 kcal mol −1 higher in energy. For molecules such as these two containing a saturated alkyl substituent, electronic contributions to the conformations may be assumed to be minimal, and steric factors should be the major structure-determining force. This conclusion is supported by the results for a number of (halomethyl)-substituted threemembered ring compounds (shown in Table 1 and Table 2), where, once again, the gauche structure (or structures) is (are) clearly preferred by these molecules, no doubt as a consequence of steric factors. In order to examine this hypothesis further, we have recorded the infrared and Raman spectra of the compound (aminomethyl)cyclopropane hydrochloride (AMCP–HCl) and its N-d 3 isotopomer. These two compounds contain the cyclopropylmethyl ammonium (CPMA) cation, which, while containing a positive charge, is isoelectronic with both ethyloxirane and ethylcyclopropane. Thus the effect of a different electronic environment on the conformational behavior of these similar molecules could be explored.

Apparently, no previous vibrational studies have been performed for the CPMA ion, so this work appears to be novel in that regard. Furthermore, very few previous conformational studies, which combine both vibrational spectroscopy and computational approaches, have been carried out for substituted three-membered ring species which are not electrically neutral. We are aware of just two such studies: on the cyclopropanecarboxylate anion [8] and the cyclopropylcarbamidinium cation [8]. In both these cases the unsaturated nature of the substituents allows for possible electronic interaction (i.e. conjugation) between the substituent and the cyclopropane ring orbitals. Therefore, in order to document more fully the conformational properties of alkyl or alkyl-like substituents attached to three-membered rings, and to investigate for the first time the effect of a charged (but saturated) substituent on the potential conformational equilibria, we have undertaken an experimental and theoretical conformational analysis of the CPMA cation found in the salt, AMCP–HCl. The results of the study are reported herein.

2. Experimental AMCP–HCl was purchased from Aldrich Chemical Company at a stated purity of 99%, and was used without further purification. The deuterated analog, AMCP–N-d 2 –DCl, was prepared by dissolving AMCP–HCl in D 2O and allowing deuterium exchange to take place. The D 2O was removed under vacuum, and the process repeated two more times. After three such deuterium exchanges, we could detect no infrared peaks associated with N–H stretches, so we assumed that the AMCP–N-d 2 –DCl

Table 2 Conformational behavior of some oxiranyl–CH 2 –X compounds X

Relative abundance of conformers (%) gauche-1

–F –Cl –Br –I

53 70 72 70

gauche-2 21 21 18 30

Reference cis 26 9 10 —

[7] [3] [4] [5]

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

thus formed was at least 95% or more isotopically pure. All subsequent operations involving the deuterated compound were conducted to minimize exposure to water vapor to prevent back-exchange. When not in use, all samples were maintained in the dark at 58C to help prevent any possible decomposition. Raman spectra of polycrystalline AMCP–HCl, amorphous AMCP–N-d 2 –DCl, a 1.0 M aqueous solution of AMCP–HCl, and 1.8 M AMCP–N-d 2 –DCl in D 2O, were excited in the 908 scattering geometry with the 514.5 nm line of an argon-ion laser (Coherent Innova-70) using 0.3 W and 0.6 W of radiant power for the solid and liquid phases respectively. Solution samples were sealed in glass capillary tubes (Kimax #34507) and maintained at room temperature, whereas the spectrum of the polycrystalline solid was recorded after crystals were ground and then sealed in the thinner end of a Pasteur pipette (Fisher Scientific, 13-678-6B, disposable/flint glass). Spectra in the region between 200 and 4000 cm −1 were recorded on a Spex Ramalog-V/VI spectrometer interfaced with an IBM computer. The instrument was calibrated with carbon tetrachloride and indene. Reported frequencies are expected to be accurate to within 1 cm −1. Spectral slit widths of 4.0 cm −1 and 8.0 cm −1, for liquid and solid respectively, were used. Parallel and perpendicular components of the polarized Raman spectra of the 1 M aqueous solution of AMCP–HCl were collected with a Spex Ramalog 1401 spectrometer, which was also calibrated with indene as a standard. A polarized analyzer and a polarization scrambler were located between the sample and the monochromator. A spectral slit width of two wavenumbers was maintained for these Raman spectra. The FT-IR spectrum of AMCP–HCl as a KBr wafer was obtained from 600 to 4000 cm −1 with a Mattson Instruments Sirius 100 spectrometer. The instrument was equipped with the standard high intensity source, a KBr beam splitter and a liquid-nitrogencooled MCT detector. Dry nitrogen was purged through the spectrometer during the data collection. No baseline corrections were made.

3. Computational work The predicted geometric parameters and the

161

frequencies for the two possible conformers of the CPMA ion were obtained using both semi-empirical and ab initio calculations. First, a low-level geometry optimization was achieved by using the AM1 semiempirical method [9] with AMPAC Version 4.5 (q 1992, SemiChem, 7128 Summit, Kansas City, KS 66216, USA). The optimized geometry thus obtained was then used as the starting point in the subsequent LCAO-MO-SCF restricted Hartree–Fock calculations, employing Gaussian 92 [10], with both the 631G* basis set and incorporating electron correlation at the MP2 level. The final optimized geometry was used as a set of fixed parameters in AMPAC Version 5.0 to obtain reasonable values for the enthalpies of formation of the two conformers. In order to obtain a more complete description of the normal vibrations of the CPMA ion’s conformers, and to help support our vibrational assignment, a normal coordinate calculation was also performed. The force field determined by the Gaussian 92 program at the MP2/6-31G* level, coupled with the internal coordinates shown in Fig. 1, formed the starting point for the normal coordinate calculation, and a scaling factor of 0.9 was used for all vibrations to calculate the vibrational frequencies. The B-matrix, created by transforming the Cartesian coordinates into the internal coordinates, and its transpose were used to transform the Cartesian force field into one described by internal coordinates in the FXP program [11] of Zhao and Durig. The FXP program also determined the potential energy distribution (PED) among a set of symmetry coordinates for each conformer (Table 3 and Table 4) and, using the predicted dipole moment and polarizability derivatives from the Gaussian 92 output, simulated the infrared and Raman spectra for the two CPMA structures.

4. Results The infrared and Raman spectra of AMCP–HCl are shown in Fig. 2, and the Raman spectrum of this compound in a 1.0 M aqueous solution is shown in Fig. 3. Similarly, Fig. 4 displays the Raman spectrum of solid AMCP–N-d 2 –DCl, and Fig. 5 contains the Raman spectrum of this heavy compound in a 1.8 M D 2O solution. These spectroscopic results are tabulated in Table 5 and Table 6 respectively, along

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Fig. 1. Internal coordinates of the CPMA cation, shown in the gauche conformer. For purposes of clarity, not all internal coordinates are shown. Refer to Tables 3 and 4 for a complete list of internal coordinates.

with the predicted spectroscopic data for the gauche conformer of the CPMA cation present in the light and heavy compounds. Table 7 and Table 8 include the predicted spectroscopic results for the cis conformation—which is not observed experimentally. Table 8 shows the predicted structural and energetic parameters for the CPMA cation, and Fig. 6 exhibits the simulated infrared and Raman spectra of this ion, in both of its conformeric possibilities for both the light and heavy isotopic versions.

5. Discussion of the spectroscopic results The majority of spectral peaks observed in the CPMA cation arise from cyclopropyl and methylene group vibrations. From this, we have been able to assign those bands due to these moieties with a fair degree of confidence. For example, the cyclopropyl group frequencies identified previously [12] enable us easily to characterize those bands in CPMA arising

from ring CH and CH 2 stretchings and bendings, as well as the three ring stretching or deformation modes. Similarly, the six vibrational modes from the –CH 2 – ‘‘spacer’’ group all fall within expected ranges. These bands are also in excellent agreement with the vibrational assignments for the corresponding modes in the isoelectronic analog: ethylcyclopropane [2]. For the most part, the cyclopropyl and methylene modes are also in good agreement with those predicted from the ab initio calculations. The major exception occurs (see Table 5) for the symmetric ring deformation mode, which we assign at 877 cm −1, but which is predicted to fall at 758 cm −1. However, the PED shown in Table 5 indicates that this mode, as well as others in this region, is heavily mixed, and the assignment of the vibration at 877 cm −1 can only be described as a ‘‘symmetric ring deformation’’ very tentatively. For consistency with previous assignments of this mode in other cyclopropane derivatives [12] we prefer to retain the

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

163

Table 3 Symmetry coordinates of the gauche-conformer of the CPMA cation

A

a

Description

Symmetry coordinates a

NH 3 stretch, asym. NH 3 stretch, asym. NH 3 stretch, sym. ring CH 2 stretch, asym., i. p. ring CH 2 stretch, asym., o. p. CH 2 stretch, asym. ring CH 2 stretch, sym., o. p. CH stretch ring CH 2 stretch, sym., i. p. CH 2 stretch, sym. NH 3 deformation, asym. NH 3 deformation, asym. ring CH 2 deformation, i. p. NH 3 deformation, sym. CH 2 deformation ring CH 2 deformation, o. p. CH bend, in plane CH 2 wag CH 2 twist ring breathing ring CH 2 twist, i. p. ring CH 2 twist, o. p. CH bend, out-of-plane CH 2 rock ring CH 2 wag, o. p. ring CH 2 wag, i. p. NH 3 rock C 1 –C 2 stretch ring deformation C–N stretch NH 3 rock, asym. ring CH 2 rock, o. p. ring CH 2 rock, i. p. ring deformation C–C–N bend –CH 2(NH 3) bend, in-plane –CH 2(NH 3) bend, out-of-plane NH 3 torsion asymmetric torsion

S 1 = 2a 8 − a 9 − a 10 S 2 = a 9 − a 10 S 3 = a 8 + a 9 + a 10 S4 = a1 − a2 + a3 − a4 S5 = a1 − a2 − a3 + a4 S6 = a6 − a7 S7 = a1 + a2 + a3 + a4 S8 = a5 S9 = a1 + a2 − a3 − a4 S 10 = a 6 + a 7 S 11 = 2h 1 − h 2 − h 3 S 12 = h 2 − h 3 S13 = 4J1 − b 1 − p 1 − b 3 − p 3 + 4J 2 − b 2 − p 2 − b 4 − p 4 S14 = h1 + h2 + h 3 − d 1 − d 2 − d 3 S15 = 4J3 − « 1 − « 2 − « 3 − « 4 S16 = 4J1 − b 1 − p1 − b 3 − p3 − 4J 2 + b2 + p 2 + b 4 + p 4 S17 = 4g − a 1 − a 2 − v 1 − v 2 S18 = « 1 + « 2 − « 3 − « 4 S19 = «1 − « 2 − « 3 + « 4 S 20 = R 1 + R 2 + R 3 S21 = b1 − p1 − b3 + p3 + b 2 − p 2 − b 4 + p 4 S22 = b1 − p1 − b3 + p3 − b2 + p 2 + b 4 − p 4 S23 = a1 + a2 − v 1 − v 2 S24 = «1 − «2 + «3 − « 4 S25 = b1 − p1 + b3 − p3 − b2 + p2 − b 4 + p 4 S26 = b1 − p1 + b3 − p3 + b 2 − p 2 + b 4 − p 4 S 27 = 2d 1 − d 2 − d 3 S 28 = R 4 S 29 = R 2 − R 3 S 30 = R 5 S 31 = d 2 − d 3 S32 = b1 + p1 − b3 − p3 − b2 − p 2 + b 4 + p 4 S33 = b1 + p1 − b3 − p3 + b 2 + p 2 − b 4 − p 4 S 34 = 2R 1 − R 2 − R 3 S35 = 5m − J3 − « 1 − « 2 − « 3 − « 4 S36 = a1 − a 2 + v 1 − v 2 S37 = a1 − a 2 − v 1 + v 2 S 38 = t 1 S 39 = t 2

Not normalized.

name ‘‘ring deformation,’’ realizing that this normal vibration (and others in this region of the spectrum) is highly mixed. Assigning the frequencies for the ammonium moiety (NH3+ ) leads to some interesting results, both experimentally and theoretically. By deuterating this group we can track which vibrations in the high frequency portion of the spectra disappear, and ‘‘reappear’’ at much lower frequencies. Interestingly,

the N–H stretching modes for the ammonium group are commingled with the (methylene) C–H stretches from 2900 to 3000 cm −1, as can readily be determined by the absence of these bands upon deuteration— where they shift to the 2200 cm −1 region. Furthermore, as Fig. 2 demonstrates, there are some characteristic multiple overtones and combination bands generated by the ammonium group between 2000 and 2800 cm −1, which are clearly observed in the

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Table 4 Symmetry coordinates of the cis-conformer of the CPMA cation Symmetry coordinates a

Description A9

NH 3 stretch, asym. NH 3 stretch, sym. ring CH 2 stretch, sym. CH stretch ring CH 2 stretch, sym. CH 2 stretch, sym. NH 3 deformation, asym. ring CH 2 deformation, i. p. NH 3 deformation, sym. CH 2 deformation CH bend, in-plane CH 2 wag ring breathing ring CH 2 twist, i. p. ring CH 2 wag, i. p. NH 3 rock ring deformation C–N stretch ring CH 2 rock, i. p. C 1 –C 2 stretch C–C–N bend –CH 2(NH 3) bend, in-plane

S 1 = 2a 8 − a 9 − a 10 S 2 = a 8 + a 9 + a 10 S3 = a1 − a2 + a3 − a4 S4 = a5 S5 = a1 + a2 + a3 + a4 S6 = a6 + a7 S 7 = 2h 1 − h 2 − h 3 S8 = 4J1 − b1 − p1 − b3 − p3 + 4J 2 − b2 − p 2 − b 4 − p 4 S9 = h1 + h2 + h3 − d 1 − d 2 − d 3 S10 = 4J3 − «1 − «2 − « 3 − « 4 S11 = 4g − a1 − a2 − v1 − v2 S12 = «1 + « 2 − « 3 − « 4 S 13 = R 1 + R 2 + R 3 S14 = b1 − p1 − b3 + p3 + b 2 − p 2 − b 4 + p 4 S15 = b1 − p1 + b3 − p3 + b 2 − p 2 + b 4 − p 4 S 16 = 2d 1 − d 2 − d 3 S 17 = 2R 1 − R 2 − R 3 S 18 = R 5 S19 = b1 + p1 − b3 − p3 + b 2 + p 2 − b 4 − p 4 S 20 = R 4 S21 = 5m − J3 − «1 − « 2 − « 3 − « 4 S22 = a1 − a 2 + v 1 − v 2

A0

NH 3 stretch, asym. ring CH 2 stretch, asym. CH 2 stretch, asym. ring CH 2 stretch, asym. NH 3 deformation, asym. ring CH 2 deformation, o. p. CH 2 twist ring CH 2 rock, o. p. CH bend, out-of-plane ring CH 2 twist, o. p. ring CH 2 wag, o. p. NH 3 rock,asym. ring deformation CH 2 rock –CH 2(NH 3) bend, out-of-plane NH 3 torsion asymmetric torsion

S 23 = a 9 − a 10 S 24 = a 1 − a 2 − a 3 + a 4 S 25 = a 6 − a 7 S 26 = a 1 + a 2 − a 3 − a 4 S 27 = h 2 − h 3 S28 = 4J1 − b1 − p1 − b3 − p3 − 4J2 + b 2 + p 2 + b 4 + p 4‘ S29 = «1 − «2 − «3 + « 4 S30 = b1 + p1 − b3 − p3 − b 2 − p 2 + b 4 + p 4 S31 = a1 + a2 − v 1 − v 2 S32 = b1 − p1 − b3 + p3 − b 2 + p 2 + b 4 − p 4 S33 = b1 − p1 + b3 − p3 − b 2 + p 2 − b 4 + p 4 S 34 = d 2 − d 3 S 35 = R 2 − R 3 S36 = «1 − «2 + «3 − «4 S37 = a1 − a2 − v 1 + v 2 S 38 = t 1 S 39 = t 2

a

Not normalized.

infrared spectrum, but which can also be seen in the Raman spectrum as much weaker bands. This pattern has been observed for other organic ammonium salts, and is thought to arise from combinations with a lowlying vibration (such as the torsion of the ammonium group) and other overtone or sum levels in this frequency range [13]. Other ammonium vibrational frequencies in the light

compound can also be identified by their shifts upon deuteration. The deformations and rocks are assigned by the disappearance (on deuteration) of bands in the spectrum of AMCP–HCl in the 1500 to 1600 cm −1 and the 1100 cm −1 regions respectively. Unfortunately, in the heavy compound, these bands become too weak to observe directly, no doubt due to the reduction in their amplitude of vibration upon deuteration.

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

165

Fig. 2. (a) Raman spectrum (bottom) of polycrystalline AMCP–HCl and infrared spectrum (top) of a KBr pellet of AMCP–HCl from 200 to 1750 cm −1. (b) Raman spectrum (bottom) of polycrystalline AMCP–HCl and infrared spectrum (top) of a KBr pellet of AMCP–HCl from 2000 to 3500 cm −1.

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Fig. 3. Raman spectrum of a 1.0 M solution of AMCP–HCl in H 2O.

Fig. 4. Raman spectrum of solid AMCP–N-d 2 –DCl.

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

167

Fig. 5. Raman spectrum of a 1.8 M solution of AMCP–N-d 2 –DCl in D 2O.

Perhaps more interesting, however, are the predicted frequencies of bands associated with the ammonium moiety. The N–H stretching frequencies are calculated to fall (even with a scaling factor of 0.9) in the 3200–3300 cm −1 region, but there are no bands observed in this region. Instead, we have confidently assigned the ammonium N–H stretches to bands nearly 300 cm −1 lower, based on our isotopic substitution results. We feel that this discrepancy arises from the fact that the ab initio calculations are carried out for the ‘‘free’’ CPMA ion, whereas the experimental spectra are recorded with the counter-ion (chloride) also present. The effect of the counter-ion would be to affect the charge in the vicinity of the ammonium moiety, thereby affecting both bond orders and, hence, vibrational frequencies. We also believe that the counter-ion influences the structural preferences of this ion in a similar fashion, as will be discussed later. The other major difference between the experimental assignment and that predicted from the quantum mechanical calculations arises for the two NH3+ rocking modes. We assign these two modes as being nearly degenerate near 1150 cm −1, as they would be

under idealized C 3v local symmetry. The calculations predict one of these rocking modes to fall in this region, but the other is predicted to fall almost 300 cm −1 lower. Once again, the modes in this region are extensively mixed, so the attribution of this mode to an ammonium rock is strictly tentative, but perhaps this mode is also influenced by the counter-ion. Oddly enough, however, the NH3+ deformation modes are predicted to fall almost exactly where they are observed experimentally. We have no ready explanation for this perhaps fortuitous result.

6. Discussion of the structural results Experimentally, all the vibrational bands observed can be assigned as arising from a single conformation of the CPMA cation; we observe no conformer doublets (or multiplets) for any peak in either the infrared or Raman spectra. Furthermore, the relatively small number of bands we observe to be depolarized in the Raman spectrum of the 1.0 M solution of AMCP–HCl could result from local symmetry effects, so we cannot conclude unequivocally that they arise from a

Fundamental NH 3 stretch, asym. NH 3 stretch, asym. NH 3 stretch, sym. ring CH 2 stretch, asym., i. p. ring CH 2 stretch, asym., o. p. CH 2 stretch, asym. ring CH 2 stretch, sym., o. p. CH stretch ring CH 2 stretch, sym., i. p. CH 2 stretch, sym. NH 3 deformation, asym. NH 3 deformation, asym. ring CH 2 deformation, i. p. NH 3 deformation, sym. CH 2 deformation ring CH 2 deformation, o. p. CH bend, in plane CH 2 wag CH 2 twist ring breathing ring CH 2 twist, i. p. ring CH 2 twist, o. p. CH bend, out-of-plane NH 3 rock, sym.

Vib. no.

n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13

n 14 n 15 n 16 n 17 n 18 n 19 n 20 n 21

n 22 n 23

n 24 1142

1240 1177

1557 1551 1526 1482 1414 1374 1277 1268

3528 3523 3411 3321 3302 3229 3223 3216 3201 3151 1724 1712 1572

Ab initio a

1084

1177 1117

1477 1472 1448 1406 1342 1304 1211 1203

3347 3342 3236 3150 3133 3064 3058 3051 3037 2990 1635 1625 1492

Fixed b

16.0

1.1 5.3

137.2 48.9 4.5 21.6 12.3 8.0 11.1 4.2

110.1 110.3 49.0 0.0 0.4 0.7 0.7 0.4 2.9 1.5 43.2 58.6 8.4

IR int.

3.3

6.3 1.6

3.4 5.9 7.9 6.4 4.1 4.8 20.2 8.1

40.5 34.4 66.8 51.9 71.1 34.9 114.9 65.4 68.1 81.2 7.1 7.2 4.1

Raman act.

0.28

0.75 0.43

0.75 0.70 0.74 0.31 0.56 0.71 0.23 0.13

0.74 0.67 0.02 0.63 0.69 0.21 0.12 0.72 0.14 0.08 0.74 0.74 0.51

DP

Table 5 Observed and calculated frequencies and potential energy distribution for gauche AMCP–HCl

1167w 1112, 1105vw 1152

2956s 2956s 2905s 3096m 3085m 2990s 3016vs 3031s 3016vs 2965s 1609vw 1609 1460, 1452vw 1493w 1419vw 1403 1343ms 1320w 1222w 1204vs 1173w

AMCP–HCl c

1154w,p

1107vw,p

1470w 1433w,dp 1412w,p 1353w,p 1323vw,dp — 1206vs,p 1176w,sh,dp

2936w,p 2936w,p 2905w,p 3097m,dp 3085m,p — 3016vs,p 3033vs,p 3016vs,p 2972s,p 1600vvw 1600vvw —

AMCP–HCl d

1165vw 1105, 1110mw 1146m

1495 1419m 1406m 1346mw — 1220w 1203vw 1173vw

2944vs 2944vs 2903vs — 3087s,sh — 3011vs,sh — 3011vs,sh — 1604ms 1604ms 1463w,p

AMCP–HCl e

8S 24, 25S 27, 13S 22

73S 14, 24S 15 71S 15, 23S 14 98S 16 33S 17, 23S 18, 13S 28 61S 18, 17S 17 60S 19, 16S 31 43S 20, 29S 17, 14S 28 25S 21, 16S 24, 13S 20, 10S 36, 10S 33 48S 22, 44S 32 56S 23, 17S 22

51S 1, 49S 2 51S 2, 49S 1 100S 3 75S 4, 23S 5 73S 5, 22S 4 50S 6, 49S 8 57S 7, 38S 9 48S 8, 48S 6 57S 9, 38S 7 100S 10 89S 11 88S 12 75S 13, 13S 20

PED

168 M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

ring deformation, asym. C–N stretch NH 3 rock, asym. ring CH 2 rock, o. p. ring CH 2 rock, i. p. ring deformation, sym. C–C–N bend –CH 2(NH 3) bend, in-plane –CH 2(NH 3) bend, out-of-plane NH 3 torsion asymmetric torsion

n 29 n 30 n 31

n 32

n 33 n 34 n 35 n 36 n 37 n 38 n 39 825 799 462 374 236 217 126

876

959 926 880

1141 1113 1052 1011

783 758 438 355 224 205 119

831

909 878 835

1082 1056 998 959

9.6 15.6 8.4 1.7 3.7 3.5 7.9

7.2

7.8 17.0 5.6

7.2 13.9 12.4 21.6

2.2 11.5 2.9 1.1 0.0 0.2 0.2

4.2

11.7 8.9 4.8

0.5 0.8 1.8 7.5

0.50 0.44 0.49 0.17 0.75 0.26 0.11

0.75

0.73 0.54 0.49

0.35 0.14 0.43 0.71

768ms 877 466, 459ms 359ms 267m — —

785ms

913s 988w 1145w

1041 1018vw 841m 941s

769m,p 879w,p 466w,p 364w,p 262vvw

794w,dp

918m,p 992w,p 1142w,p

1052vw,p 1031vvw,dp 836m,dp 943m,p

762mw 871w 469ms — — — —

785s

912s 983s 1138m

— 1017s 836mw 935vw

94S 25 90S 26 25S 27, 12S 31, 20S 28, 31S 34, 10S 33 62S 29 55S 30, 15S 34 18S 31, 22S 24, 17S 30 30S 32, 19S 23, 14S 22 36S 33, 20S 21, 28S 34, 23S 21, 39S 35, 30S 37 75S 36 26S 37, 34S 38, 57S 38, 23S 37, 79S 39 24S 24 13S 35

19S 24 16S 28

15S 29,

20S 33,

10S 34 16S 21,

Abbreviations: i. p. = in phase; o. p. = out of phase; sym. = symmetric; asym. = asymmetric; m = medium, s = strong, w = weak, vs, vw, etc.; for very strong and very weak bands. br = broad and sh = shoulder. a Calculated using the MP2/6-31G* basis set. b Calculated using a scaling factor of 0.9 for all the vibrations. c From the polycrystalline solid phase Raman spectrum. d From the Raman spectrum of a 1 M aqueous solution. e FTIR spectrum of a KBr wafer.

ring CH 2 wag, o. p. ring CH 2 wag, i. p. CH 2 rock C 1 –C 2 stretch

n 25 n 26 n 27 n 28

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169

ring CH 2 stretch, asym., i. p. ring CH 2 stretch, asym., o. p. CH 2 stretch, asym. ring CH 2 sym. stretch, o.p. CH stretch ring CH 2 stretch, sym., i. p. CH 2 stretch sym. ND 3 stretch, asym. ND 3 stretch, asym. ND 3 stretch, sym. ring CH 2 deformation, i. p. CH 2 deformation ring CH 2 deformation, o. p. CH bend, in-plane CH 2 wag CH 2 twist ring breathing CH 2 rock ring CH 2 twist, o. p. ND 3 deformation, asym. ND 3 deformation, asym. ND 3 deformation, sym. CH bend, out-of-plane ring CH 2 wag, o. p. ring CH 2 wag, i. p. C 1 –C 2 stretch ring deformation, asym. ring CH 2 twist, i. p. ring deformation, sym. ring CH 2 rock, o. p. C–N stretch ring CH 2 rock, i. p. ND 3 rock, sym. ND 3 rock, asym. –CH 2(ND 3) bend, out-of-plane –CH 2(ND 3) bend, in-plane C–C–N bend ND 3 torsion asymmetric torsion

n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 n 16 n 17 n 18 n 19 n 20 n 21 n 22 n 23 n 24 n 25 n 26 n 27 n 28 n 29 n 30 n 31 n 32 n 33 n 34 n 35 n 36 n 37 n 38 n 39 3320.6 3302.3 3229.3 3223.3 3216.3 3200.7 3151.6 2602.8 2599.2 2443.7 1571.7 1552.4 1526.7 1480.4 1404.5 1344.5 1276.6 1250.6 1243.8 1237.3 1233.0 1195.8 1172.8 1140.5 1114.4 1087.5 1002.8 975.6 917.4 879.2 869.0 823.6 737.2 716.9 437.2 368.4 218.6 162.2 118.0

Ab initio a

3150.1 3132.7 3063.5 3057.8 3051.1 3036.5 2989.7 2469.0 2465.7 2318.2 1491.1 1472.8 1448.3 1404.4 1332.5 1275.6 1211.1 1186.4 1180.0 1173.8 1169.8 1134.5 1112.3 1082.0 1057.2 1031.7 951.3 925.5 870.4 834.1 824.5 781.3 699.4 680.2 414.8 349.5 207.5 154.1 112.0

Fixed b

See Table 5 for a list of abbreviations used. a Calculated using the MP2/6-31G* basis set. b Calculated using a scaling factor of 0.9 for all the vibrations. c Raman spectrum of an approximately 1.8 M solution in D 2O.

Fundamental

Vib. no. A

Table 6 Observed and calculated frequencies and PED for gauche-AMCP–N-d 2 –DCl

0.1 0.4 0.5 0.6 0.6 3.0 1.2 58.0 60.5 31.8 3.5 9.7 8.5 10.4 8.1 1.7 8.3 1.3 13.3 15.3 23.7 99.8 8.8 9.0 7.8 14.0 23.4 1.8 7.5 6.1 1.9 0.1 9.6 12.1 7.0 1.7 4.3 0.9 5.2

IR Int.

51.8 71.1 34.6 114.9 63.6 67.7 78.5 22.5 16.8 33.8 5.0 8.1 7.9 6.7 3.7 6.3 22.9 5.4 5.4 0.9 4.9 2.0 1.5 0.5 0.3 7.2 9.8 2.6 5.4 8.1 10.2 1.4 8.1 2.0 2.1 1.2 0.1 0.1 0.1

Ram.Act.

0.63 0.68 0.21 0.12 0.72 0.14 0.08 0.73 0.70 0.02 0.54 0.71 0.73 0.32 0.61 0.70 0.20 0.28 0.69 0.72 0.70 0.15 0.63 0.52 0.50 0.30 0.74 0.74 0.44 0.72 0.72 0.75 0.39 0.36 0.50 0.17 0.44 0.19 0.12

DP

3099m 3087 — 3022vs — 3022vs 2974s 2210br 2210br 2184br 1459, 1453m 1434w 1409 1445w 1293w — 1203vs 833s 1185 — — — 1110vw — 1038ms 946w 906s — 856s 752m 978mw 732m — — 441m 367m

Exp. c

75S 1, 23S 2 73S 2, 22S 1 50S 3, 49S 5 56S 4, 38S 6 48S 5, 48S 3 57S 6, 38S 4 100S 7 61S 8, 39S 9 61S 9, 39S 8 100S 10 78S 11 94S 12 98S 13 36S 14, 19S 15, 13S 26, 11S 11 72S 15, 14S 14 72S 16 51S 17, 29S 14 19S 18, 19S 28, 11S 26, 11S 20 21S 19, 20S 30, 30S 20 53S 20, 22S 19, 19S 30 85S 21 87S 22 61S 23, 24S 19 93S 24 89S 25 14S 26, 21S 18 34S 27, 14S 26, 14S 29 12S 28, 21S 32, 16S 18 44S 29, 37S 31 19S 30, 23S 27 26S 31, 19S 29, 18S 30, 12S 27, 11S 23 56S 32, 34S 28 42S 33, 13S 26, 12S 31 60S 34, 20S 18 35S 35, 32S 37 78S 36 39S 37, 40S 35 90S 38 78S 39

PED

170 M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

3530 3296 3215 3196 1719 1537 1387 1229 1200 1158 1127 963 893 826 358 241 81

939 813 763 505 259

3527 3418 3306 3233 3203 3147 1725 1587 1563 1557 1469 1454 1285 1180 1128 1090 1017

Ab initio a

See Table 5 for a list of abbreviations used. a Calculated using the MP2/6-31G* basis set. b Calculated using a scaling factor of 0.9 for all the vibrations.

NH 3 stretch, asym. ring CH 2 stretch, asym. CH 2 stretch, asym. ring CH 2 stretch, asym. NH 3 deformation, asym. ring CH 2 deformation, o. p. CH 2 twist ring CH 2 rock, o. p. CH bend, out-of-plane ring CH 2 twist, o. p. ring CH 2 wag, o. p. NH 3 rock, asym. ring deformation CH 2 rock –CH 2(NH 3) bend, out-of-plane NH 3 torsion asymmetric torsion

C–N stretch ring CH 2 rock, i. p. C 1 –C 2 stretch C–C–N bend –CH 2(NH 3) bend, in-plane

NH 3 stretch, asym. NH 3 stretch, sym. ring CH 2 stretch, sym. CH stretch ring CH 2 stretch, sym. CH 2 stretch, sym. NH 3 deformation, asym. ring CH 2 deformation, i. p. NH 3 deformation, sym. CH 2 deformation CH bend, in-plane CH 2 wag ring breathing ring CH 2 twist, i. p. ring CH 2 wag, i. p. NH 3 rock ring deformation

A9 n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 n 16 n 17

n 18 n 19 n 20 n 21 n 22 A0 n 23 n 24 n 25 n 26 n 27 n 28 n 29 n 30 n 31 n 32 n 33 n 34 n 35 n 36 n 37 n 38 n 39

Fundamental

Vib. no.

Table 7 Calculated frequencies and PED for cis-AMCP–HCl

3348.7 3127.3 3050.3 3032.3 1630.4 1457.9 1314.5 1165.5 1137.9 1098.4 1069.0 913.2 847.1 783.6 339.4 228.2 76.8

890.4 771.2 723.7 479.4 245.3

3346.3 3242.9 3136.4 3067.8 3038.8 2985.9 1636.9 1505.1 1482.5 1477.3 1393.9 1379.2 1219.4 1119.5 1069.7 1034.4 964.8

Fixed b

96.7 1.1 0.1 4.4 71.9 4.8 14.5 1.7 3.5 0.2 5.4 12.1 1.6 11.4 1.4 0.3 4.3

11.3 0.7 6.0 8.7 22.8

134.3 40.9 1.6 1.1 20.0 3.1 54.7 14.2 110.7 66.8 13.3 12.5 1.9 7.9 5.9 42.4 0.5

IR int.

19.1 78.5 68.7 22.1 6.0 8.9 5.4 6.7 0.7 0.5 0.4 11.9 5.4 2.0 0.6 0.0 0.1

8.9 2.7 11.1 1.6 0.5

39.0 59.5 50.5 75.3 138.8 99.7 4.7 3.2 3.7 6.3 5.0 2.5 21.5 1.0 4.3 4.8 7.7

Raman act.

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.46 0.75

0.42 0.34 0.43 0.69 0.09

0.69 0.02 0.37 0.29 0.05 0.09 0.73 0.64 0.69 0.73 0.42 0.70 0.15 0.75 0.21 0.23 0.71

DP

100S 23 90S 24 100S 25 91S 26 94S 27 99S 28 71S 29, 17S 34 46S 30, 40S 32 39S 31, 24S 36, 14S 34 28S 32, 23S 31, 14S 36 89S 33 31S 34, 46S 35 40S 35, 17S 30, 11S 34 29S 36, 25S 30, 17S 31, 13S 34, 12S 32 83S 37 90S 38 92S 39

100S 1 100S 2 92S 3 99S 4 92S 5 100S 6 94S 7 74S 8, 15S 13 59S 9, 37S 10 61S 10, 37S 9 37S 11, 17S 8, 17S 20, 12S 13 82S 12 46S 13, 32S 11 31S 14, 18S 19, 14S 16 53S 15, 12S 17, 10S 16 25S 16, 43S 15 20S 17, 22S 16, 17S 18, 11S 14, 10S 20, 10 S 19 59S 18, 26S 17 60S 19, 17S 14 30S 20, 34S 17, 19S 14 39S 21, 35S 22 52S 22, 43S 21

PED

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

171

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M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

Table 8 Calculated frequencies and PED for cis-AMCP–N-d 2 –DCl Vib. # A9 n1 n2 n3 n4 n5 n6 n7 n8 n9 n 10 n 11 n 12 n 13 n 14 n 15 n 16 n 17 n 18 n 19 n 20 n 21 n 22 A0 n 23 n 24 n 25 n 26 n 27 n 28 n 29 n 30 n 31 n 32 n 33 n 34 n 35 n 36 n 37 n 38 n 39

Fundamental

Ab initio a

Fixed b

IR Int.

Ram. Act.

DP

ring CH 2 stretch, sym. CH stretch ring CH 2 stretch, sym. CH 2 stretch, sym. ND 3 stretch, asym. ND 3 stretch, sym. ring CH 2 deformation, i. p. CH 2 deformation CH bend, i. p. CH 2 wag ring breathing ND 3 deformation, asym. ND 3 deformation, sym. ring CH 2 twist, i. p. ring CH 2 wag, i. p. C 1 –C 2 stretch C–N stretch ring deformation ring CH 2 rock, i. p. ND 3 rock –CH 2(ND 3) bend, in-plane C–C–N bend

3305.9 3234.1 3203.0 3147.7 2601.6 2449.8 1586.2 1559.1 1469.2 1444.4 1285.7 1240.8 1209.6 1158.3 1118.8 1057.4 931.5 864.0 807.3 728.9 471.4 241.3

3136.4 3067.8 3038.8 2985.9 2468.1 2324.0 1504.8 1479.1 1393.8 1370.3 1219.8 1177.1 1147.5 1098.8 1061.4 1003.2 883.7 819.7 765.8 691.5 447.1 228.9

1.5 1.1 19.7 2.5 74.4 26.6 3.5 9.2 3.7 9.5 0.6 28.3 95.3 6.2 6.7 20.3 1.7 5.2 0.1 5.3 5.3 18.9

49.6 75.4 140.1 97.3 19.9 29.7 3.3 9.4 5.1 3.1 21.0 2.1 0.6 1.2 1.9 12.6 4.1 10.3 2.1 8.0 1.3 0.4

0.38 0.29 0.05 0.10 0.70 0.02 0.64 0.72 0.38 0.74 0.14 0.69 0.40 0.28 0.24 0.33 0.30 0.73 0.47 0.34 0.62 0.10

92S 1 99S 2 92S 3 100S 4 100S 5 100S 6 76S 7 99S 8 43S 9, 18S 7, 14S 11, 13S 16 87S 10 45S 11, 32S 9 96S 12 73S 13 29S 14, 20S 19, 15S 13, 13S 9 81S 15 22S 16, 23S 18, 17S 15, 16S 11 51S 17, 12S 20, 10S 19 46S 18, 37S 20 62S 19, 24S 14 20S 20, 29S 16, 15S 17, 14S 14 39S 21, 28S 22, 15S 20 48S 22, 46S 21

ring CH 2 stretch, asym. CH 2 stretch, asym. ring CH 2 stretch, asym. ND asym. stretch ring CH 2 deformation, o. p. CH 2 twist ND 3 deformation, asym. ring CH 2 twist, o. p. CH bend out-of-plane CH 2 rock ring CH 2 wag, o. p. ring deformation ring CH 2 rock, o. p. ND 3 rock, asym. –CH 2(ND 3) bend, out-of-plane ND 3 torsion asymmetric torsion

3296.3 3215.6 3196.1 2603.7 1538.6 1352.7 1239.1 1225.8 1183.1 1136.6 1125.7 923.0 860.0 718.4 356.1 182.4 73.2

3127.3 3050.4 3032.3 2470.1 1459.6 1283.4 1175.5 1162.9 1122.4 1078.3 1067.9 875.7 815.9 681.6 337.8 173.1 69.6

1.2 0.0 4.4 51.2 10.4 4.5 30.9 0.6 1.7 0.1 7.4 6.4 4.5 5.5 1.4 0.2 2.4

78.3 67.2 22.2 10.8 8.4 6.4 0.4 8.6 1.3 0.9 0.1 16.5 1.7 0.2 0.6 0.0 0.1

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

90S 23 100S 24 91S 25 99S 26 100S 27 87S 28 81S 29 30S 30, 40S 35, 17S 29, 10S 31 56S 31, 19S 30, 12S 32 37S 32, 22S 33 72S 33, 11S 30, 12S 32 84S 34 43S 35, 25S 31, 20S 30 70S 36, 20S 32 85S 37 86S 38, 11S 39 86S 39, 11S 38

PED

See Table 5 for a list of abbreviations used. a Calculated using the MP2/6-31G* basis set. b Calculated using a scaling factor of 0.9 for all the vibrations.

conformer having an element of symmetry, such as the cis conformation of the CPMA cation (which has C s symmetry). In order to identify the sole conformer of the CPMA ion that is present under ambient conditions, we have to look to the computational results, since the spectra

provide no concrete evidence as to which rotameric form is present. Fortunately, the calculations do provide some guidance in this area, and on two fronts. From the ab initio calculations, it is predicted that the cis conformer, while stable, is either 4.8 kcal mol −1 higher in energy than the gauche form using

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176

173

Fig. 6. Simulated vibrational spectra of: (a) infrared spectrum of the cis conformer of the CPMA ion; (b) infrared spectrum of the gauche conformer of the CPMA ion; (c) infrared spectrum of the cis conformer of the CPMA–N-d 3 ion: (d) infrared spectrum of the gauche conformer of the CPMA–N-d 3 ion; (e) Raman spectrum of the cis conformer of the CPMA ion; (f) Raman spectrum of the gauche conformer of the CPMA ion; (g) Raman spectrum of the cis conformer of the CPMA–N-d 3 ion; (h) Raman spectrum of the gauche conformer of the CPMA–N-d 3 ion.

the 6-31G* basis set, or 4.7 kcal mol −1 higher when electron correlation at the MP2 level is incorporated into the calculations. Even though these calculations are for the ‘‘free’’ CPMA ion (as noted above), we feel that they would be reasonably predictive even in the presence of the chloride counter-ion, although we have not carried out that calculation since no X-ray structural results appear to exist for AMCP–HCl to substantiate such calculations. Perhaps stronger evidence for the predominance of the gauche conformation of the CPMA cation is found in the simulated spectra, shown in Fig. 6. For both the Raman and infrared spectra, of both the light and heavy forms of the CPMA ion, the best match with the experimental spectra is clearly for the gauche conformer. From this we conclude that the gauche

conformer of the CPMA ion is the sole conformer giving rise to the observed spectra. In fact, if we examine the observed spectra for peaks which are predicted to be strong for the cis conformer, we can find no compelling evidence for the existence of this conformation from the spectral data. Thus we feel confident that the CPMA ion exists in the gauche conformation exclusively, from both theoretical and experimental considerations. Furthermore, this result is clearly consistent with those found for related molecules, like the halomethyl-cylcopropanes and oxiranes [3–7], and the two molecules which are isoelectronic with the CPMA ion: ethylcyclopropane [2] and ethyloxirane [1]. Finally, we note that the calculated structural parameters for the CPMA cation

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Table 9 Structural parameters for the CPMA cation a RHF/6-31G* Parameter Bond lengths C 1 –C 2 C 2 –C 3 C 2 –C 5 C 3 –C 5 C 1 –N 4 C 2 –H 6 C 3 –H 7 C 3 –H 9 C 5 –H 8 C 5 –H 10 C 1 –H 11 C 1 –H 12 N 4 –H 13 N 4 –H 14 N 4 –H 15 Bond angles C 3 –C 2 –C 1 C 3 –C 2 –C 5 C 5 –C 2 –C 1 N 4 –C 1 –C 2 H 6 –C 2 –C 1 H 7 –C 3 –C 2 H 8 –C 5 –C 2 H 9 –C 3 –C 2 H 10 –C 5 –C 2 H 11 –C 1 –C 2 H 12 –C 1 –C 2 H 13 –N 4 –C 1 H 14 –N 4 –C 1 H 15 –N 4 –C 1 H 7 –C 3 –H 9 H 8 –C 5 –H 10 H 11 –C 1 –H 12 H 13 –N 4 –H 14 H 13 –N 4 –H 15 H 14 –N 4 –H 15 Dihedral angles N 4 –C 1 –C 2 –C 3 C 5 –C 2 –C 1 –C 3 H 6 –C 2 –C 1 –N 4 H 7 –C 3 –C 2 –C 5 H 8 –C 5 –C 2 –C 3 H 9 –C 3 –C 2 –C 5 H 10 –C 5 –C 2 –C 3 H 9 –C 3 –C 2 –H 7 H 10 –C 5 –C 2 –H 8 H 11 –C 1 –C 2 –H 6 H 12 –C 1 –C 2 –H 6 H 13 –N 4 –C 1 –H 11 H 13 –N 4 –C 1 –C 2

gauche 1.4992 1.5098 1.5014 1.4886 1.5238 1.0777 1.0744 1.0776 1.0737 1.0754 1.0802 1.0806 1.0112 1.0108 1.0110

MP2/6-31G* cis

gauche 1.5101 1.4984 1.4984 1.4981 1.5146 1.0763 1.0740 1.0781 1.0740 1.0781 1.0806 1.0806 1.0110 1.0104 1.0104

1.4910 1.5140 1.5056 1.4946 1.5243 1.0877 1.0839 1.0872 1.0833 1.0849 1.0922 1.0922 1.0288 1.0280 1.0287

cis 1.5039 1.5024 1.5024 1.5047 1.5132 1.0870 1.0837 1.0875 1.0837 1.0870 1.0925 1.0925 1.0284 1.0280 1.0280

120.1 59.3 119.0 109.5 114.4 117.5 117.0 118.6 118.1 112.6 113.1 112.4 111.2 110.5 114.1 114.2 109.3 107.5 108.0 107.1

124.4 60.0 124.4 113.0 109.3 117.5 117.5 120.2 120.2 111.6 111.6 111.1 111.6 111.6 112.9 112.8 109.1 107.3 107.3 107.7

119.1 59.3 118.6 109.0 115.3 117.7 117.0 118.6 117.9 112.2 113.6 113.0 110.9 109.6 114.3 114.4 109.2 107.7 108.4 107.1

123.9 60.1 123.9 111.8 110.2 117.6 117.6 120.4 120.4 111.8 111.8 111.5 111.1 111.1 112.8 112.8 109.1 107.6 107.6 107.7

−79.1 −69.2 68.1 −109.0 109.4 107.0 −108.0

37.08 −37.08 180.0 −109.2 109.2

−76.1 −68.8 70.9 −108.8 109.8 106.6 −108.1

37.3 −37.29 180.0 −109.1 109.1

185.57 −49.9 55.8

−144.3 144.3 61.3 −61.3 180.0

188.0 −47.6 54.7

−144.1 144.1 61.2 −61.2 180.0

175

M. Ayub et al./Journal of Molecular Structure 449 (1998) 159–176 Table 9 (Continued) RHF/6-31G*

MP2/6-31G*

Parameter

gauche

cis

gauche

cis

H 14 –N 4 –C 1 –H 13 H 15 –N 4 –C 1 –H 13

120.6 −120.7

120.1 −120.1

121.0 −121.0

119.7 −119.7

Heat of formation (kcal mol −1) b Total energy c (Hartree) a b c

168.6420 −211.4890091 −211.4813279 DE = 4.82 kcal mol −1

170.3639 DH = 1.7219 kcal mol −1 −212.1928137 −212.1853927 DE = 4.657 kcal mol −1

For the numbering system refer to Fig. 1. Heat of formation from AMPAC 5.0 using the MP2/6-31G* level optimized geometry. 1 Hartree = 627.510 kcal mol −1.

(shown in Table 9), are in excellent agreement with the corresponding structural results for ethylcyclopropane [2]. For example, ring C–C and C–H bond lengths are remarkably consistent between these two species, as are the structural parameters for the methylene group. Only the N–H bond lengths differ significantly in CPMA when compared with their ethylcyclopropane C–H counterparts (NH3+ versus CH 3). As expected, the N–H bond lengths are substantially shorter.

7. Summary and conclusions From our experimental data and theoretical calculations, we conclude that the CPMA cation found in AMCP–HCl exists in only the gauche conformation under ambient conditions. This no doubt arises from steric considerations which tend to destabilize the cis conformation. If one factors in the chloride counterion, its presence in the vicinity of the CPMA ion would also contribute to significant steric problems for the cis conformation. In the gauche form, the chloride ion can find more room to be near the CPMA cation. Structural details, such as bond lengths and angles, were also calculated for the gauche and cis rotamers of the CPMA cation, and were found to be in excellent agreement with those for similar molecules. In addition to determining the structural preference of the CPMA ion, we have also assigned the infrared and Raman spectra of the AMCP–HCl molecule in its hydrogenated and deuterated forms, and we have successfully simulated these spectra. While we have not

published the force field for the CPMA ion here, those interested in obtaining the force constants for this species may write to the corresponding author. In sum, we have also demonstrated that the combination of experimental vibrational spectra coupled with high-level quantum mechanical calculations can prove to be a powerful tool in the structural chemist’s kit. These techniques work well even for systems such as three-membered ring compounds, where the ring strain and unusual bonding present often contribute to interesting structures, and it is possible to ascertain whether electronic or steric factors are the primary structure-determining forces in these substances. Acknowledgements The authors wish to thank Professor George J. Thomas, Jr. for allowing us to record the Raman spectra on his instruments, and Dr. Yifu Guan for assisting in their recording. We would also like to thank Professor James R. Durig for letting us use the FXP program. References [1] J.R. Durig, S. Shen, T.K. Gounev, C.J. Wurrey, J. Mol. Struct. 379 (1996) 267–282. [2] C.J. Wurrey, S. Shen, T.K. Gounev, J.R. Durig, J. Mol. Struct. 406 (1997) 207–218. [3] V.F. Kalasinsky, C.J. Wurrey, J. Raman Spectrosc. 9 (1980) 315–323. [4] C.J. Wurrey, R. Krishnamoorthi, S. Perchsiri, V.F. Kalasinsky, J. Raman Spectrosc. 12 (1982) 95–101.

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[5] C.J. Wurrey, Y.-Y. Yeh, R. Krishnamoorthi, R.J. Berry, J.E. DeWitt, V.F. Kalasinsky, J. Phys. Chem. 88 (1984) 4059– 4063. [6] C.J. Wurrey, Y.-Y. Yeh, M.D. Weakley, V.F. Kalasinsky, J. Raman Spectrosc. 15 (1984) 179–185. [7] V.F. Kalasinsky, C.J. Wurrey, J. Raman Spectrosc. 9 (1980) 45–49. [8] Patel, M.S. Thesis, University of Missouri–Kansas City, 1994 (and references cited therein). [9] M.J.S. Dewar, G.E. Zoebish, F.E. Healy, J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902–3909. [10] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel,

M.A. Robb, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Steward, J.A. Pople, Gaussian 92, Gaussian, Inc., Pittsburgh, PA, 1992. [11] W.Y. Zhao, J.R. Durig, personal communication, 1997. [12] C.J. Wurrey, A.B. Nease, Vibrational spectroscopy and structure of three membered ring compounds, in J.R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 7, Elsevier, Amsterdam, 1978, Chapter 1, pp. 1–235. [13] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, Boston, MA, 1991.