Conformational Stability from Variable-Temperature Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r o Structural Parameters of Chlorocyclopentane

The infrared spectra (3500–400 cm^–1) of krypton solutions of chlorocyclopentane, c-C₅H₉Cl, at variable temperatures (–101 to –150°C) have been recorded and the fundamental vibrations of the axial conformer and several of those for the equatorial form have been assigned. Utilizing two pairs of fundamentals for the two conformers in the krypton solution, an enthalpy difference of 145±15 cm^–1 (1.73±0.18 kJ-mol^–1) has been obtained with the axial conformer the more stable form. It is estimated that there is 67±2% of the axial conformer present at ambient temperature. Convincing spectroscopic evidence shows that a significant percentage of the chlorocyclopentane molecules are undergoing pseudorotation at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation and these quantities have been compared to the experimental values when appropriate. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2 calculations as well as by density functional theory (DFT) by the B3LYP method with several different basis sets. The adjusted r ₀ structural parameters have been obtained for both conformers by combining the ab initio data with the previously reported microwave rotational constants. These new values of the structural parameters for both conformers are compared to those previously reported from electron diffraction and microwave studies. These results are compared to the corresponding quantities of some similar molecules.     

Raman and Infrared Spectra, Ab Initio Calculations, Normal Coordinate Analysis, and Conformational Stability of 2-Methoxypropene

The Raman (3500–40 cm^–1) and infrared (3500–70 cm^–1) spectra of gaseous and solid 2-methoxypropene, CH₃O(CH₃)C=CH₂, and the isotopomers, CD₃O(CH₃)C=CH₂ and CH₃O(CD₃)C=CD₂ have been recorded. In addition, the Raman spectra of the liquids have been recorded with qualitative depolarization measurements. All of these data indicate that only one conformer is present in the fluid phases at ambient temperature and this form is the cis conformer, which remains in the solid. Assignments are provided for the fundamentals of all three isotopomers for the cis conformer with C_s symmetry. The far-infrared spectra of all three isotopic species have been recorded at a resolution of 0.1 cm^–1 in the gas and 1.0 cm^–1 in the solid. The parameters of the potential function governing the asymmetric torsion are determined to be V₃ = 1485 ± 9 cm^–1 and V₆ = –55 ± 4 cm^–1 for the d₀ compound, where only two terms were determined, since a second conformer was not evident. The barrier to internal rotation for the methyl group attached to the oxygen atom is 1370 ± 8 cm^–1 and the C—CH₃ barrier is 772 ± 5 cm^–1. Ab initio calculations with full electron correlation have been carried out by the perturbation method to second order to obtain the equilibrium structural parameters, harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, depolarization values, and conformational stability. The predicted values have been compared to the experimental values where appropriate.     

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions,Ab Initio Calculations,and r 0 Structural Parameters of the Propenoyl Halides (F,Cl, Br)

Variable temperature (–100 to –150°C) studies of the infrared spectra (3500–400 cm^–1) of propenoyl bromide, CH₂=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm^–1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm^–1, which is in excellent agreement with the experimental results. The r ₀ adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

Infrared and Raman Spectra,Conformational Stability,Ab Initio Calculations,and Vibrational Assignments for Cyclopropyldifluorosilane

The infrared (3200–30 cm^–1) spectra of gaseous and solid Cyclopropyldifluorosilane, c-C₃H₅SiF₂H, and the Raman spectra (3200–20 cm^–1) of the liquid with quantitative depolarization values and the solid have been recorded. Both the syn (cis) and skew (gauche) conformers have been identified in the fluid phases, but only the syn conformer remains in the solid. Variable temperature (–55 to –100°C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 73 ± 10 cm^–1 (209 ± 29 cal mol^–1), with the syn conformer being the more stable rotamer, which is at variance with the predictions from ab initio calculations. A complete vibrational assignment is proposed for both conformers based on infared band contours, relative intensities, depolarization values, and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G* calculations. Utilizing the frequencies of the silicon–hydrogen sketch, the rm Si—H bond distances of 1.474 and 1.472 Å have been obtained for the syn and skew conformers, respectively. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G* and 6-311 +G** basis sets at levels of restricted Hartree-Fock (RHF) and/or Moller–Plesset (MP) to second order. The potential energy terms for the conformer interconversion have been obtained from the MP2/6-31G* calculation. The results are discussed and compared to those obtained for some similar molecules.     

Conformational studies of cyclopropylmethyl isocyanate from temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3200 to 100 cm⁻¹) of cyclopropylmethyl isocyanate, c-C₃H₅CH₂NCO, dissolved in liquefied xenon, have been carried out. The infrared spectra (gas and solid) as well as the Raman spectrum of the liquid have been recorded from 3200 to 100 cm⁻¹. By analyzing six conformer pairs in xenon solutions, an enthalpy difference of 193 ± 19 cm⁻¹ (2.31 ± 0.23 kJ/mol) was obtained with the gauche–cis rotamer (the first designation indicates the orientation of the CNCO group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCO group with respect to the bridging CC bond) the more stable form and the only form present in polycrystalline solid. The abundance of the cis–trans conformer present at ambient temperature is 16 ± 1%. The potential function governing the conformational interchange has been obtained from B3LYP/6-31G(d) calculations and the two-dimensional potential has been obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche–cis conformer is predicted to be more stable by 223 to 269 cm⁻¹, which is consistent with the experimental results. However, without diffuse functions the predicted conformational energy differences are much smaller (77–166 cm⁻¹). Similar diffuse function dependency affects density functional theory calculations by the B3LYP method to a lesser extent. A complete vibrational assignment for the gauche–cis conformer is proposed and several fundamentals for the cis–trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and r₀ structural parameters are estimated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Far-Infrared Spectra, Conformational Stability, Barriers to Internal Rotation, Ab Initio Calculations, r0 Structural Parameters, and Vibrational Assignment of Ethyl Methyl Ether

The far-infrared spectra (350–35 cm^–1) of gaseous ethyl methyl ether-d ₀ and ethyl methyl-d ₃-ether have been recorded at a resolution of 0.10 cm^–1. For the d ₀ species, the fundamental asymmetric torsion of the more stable trans conformer (two methyl moieties are trans to one another) has been observed at 115.40 cm^–1 with four upper state transitions falling to lower frequency, whereas, for the gauche form, it has been observed at 93.56 cm^–1 with two excited states falling to lower frequency. the corresponding series for the d ₃ species start from 106.00 and 87.10 cm^–1, respectively. From these data, the asymmetric torsional potential coefficients for the d ₀ species have been determined to be: V ₁ = 572 ± 30; V ₂ = 85 ± 8; V ₃ = 619 ± 30; V ₄ = 175 ± 18, and V ₆ = –28 ± 3 cm^–1. The trans to gauche and gauche to gauche barriers were calculated to be 958 cm^–1 (11.5 kJ/mol) and 631 cm^–1 (7.55 kJ/mol), respectively, with an energy difference of 550 ± 6 cm^–1 (6.58 ± 0.07 kJ/mol). Utilizing three conformer pairs, variable temperature studies (–105 to –150°C) of the infrared spectra of the d ₀ sample dissolved in liquid krypton gave an enthalpy difference of 547 ± 28 cm^–1 (6.54 ± 0.33 kJ/mol) with the trans conformer the more stable rotamer. It is estimated that there is only 4% of the gauche conformer present at ambient temperatures. The structural parameters, conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies, which have been determined experimentally, are compared to those obtained from ab initio gradient predictions from RHF/6-31G* and with full electron correlation at the MP2 level with three different basis sets. The adjusted r ₀ structural parameters have been obtained for the trans conformer from combined ab initio MP2/6-311+G** predictions and previously reported microwave rotational constants. The reported distances should be accurate to 0.003 Å and the angles to 0.5°. These results are compared to the corresponding quantities obtained for some similar molecules.     

Conformational Study of Naringenin in the Isolated and Solvated States by Semiempirical and Ab Initio Methods

Naringenin is a natural widespread flavanone occurring in different foodstuffs that presents several important biological activities. Although its properties are well documented, its mechanisms of action are still controversial. The present article reports a conformational analysis of naringenin, using the semiempirical AM1 and ab initio methods, at the Hartree–Fock level of theory. The 3-21G, 3-21G*, 6-31G, and 6-31G** basis sets were used. The electron correlation effects were included through the Møller–Plesset second-order perturbation theory. The solvation of naringenin has been investigated through the standard SCRF, the supermolecule (SM), and the combined SM/SCRF models. The results have shown that there are two degenerate forms of naringenin, differing mainly by the orientation of a hydroxyl group (C4—OH). The energy barrier for the interconversion between them is ca. 6 kcal.mol^–1, suggesting some conjugation between the -system of the aromatic B ring and the hydroxyl group (C4—OH).     

Spectra and Structure of Silicon-Containing Compounds. XXVIII1 Infrared and Raman Spectra,Vibrational Assignment,and Ab Initio Calculations of Vibrational Spectrum and Structural Parameters of Vinyltrichlorosilane

The infrared spectra (3200-50 cm^–1) of gaseous and solid vinyltrichlorosilane, CH₂=CH-SiCl₃, have been recorded. In addition, the Raman spectrum (3200-10 cm^–1) of the liquid has been recorded and quantitative depolarization values obtained. The infrared spectrum of the sample dissolved in liquid xenon (–80°C) has also been recorded. Using the experimental data and normal coordinate calculations with scaled ab initio force constants, the complete vibrational assignment is proposed. The torsional mode was observed in the infrared spectrum of the gas at 69 cm^–1 and the threefold barrier of internal rotation was calculated to be 500 cm^–1 (5.98 kJ/mol). Ab initio calculations have been carried out at the restricted Hartree–Fock level of the theory as well as with full electron correlation by the perturbation method to second order with different basis sets up to 6-311+G(d,p) to obtain the optimized geometries, harmonic force constants, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies. The ab initio predicted structural parameters are compared with those obtained from a previous electron diffraction study.     

Conformational flexibility and aromaticity of azanaphthalenes

Conformational flexibility of naphthalene and its different mono-and diaza analogues has been studied by MP2/6-31G(d, p) quantum chemical computations. It is shown that all molecules possess a considerable conformational flexibility. The out-of-plane deformation energy of both a bicyclic π-system on the whole and separate rings directly depends on the degree of aromaticity of the conjugated system. Effects of the amount and arrangement of nitrogen atoms on conformational features and aromaticity of the molecules considered were analyzed.     

Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

Na2O-P2O5系晶体微结构形态的拉曼光谱研究及其ab initio计算

本文采用激光拉曼光谱仪测量了磷酸钠二元系(1-x)Na₂O·xP₂O₅(x=0.25,0.33,0.50,1.0)几种晶体的拉曼光谱,比较并解释了随化学组成而变化微结构单元的拉曼振动模。同时用Gaussian 98W量子化学软件从头计算了这几种化合物的拉曼光谱。实验和计算均表明,磷酸盐晶体的基本结构单元为磷氧四面体[PO₄],并且晶体中磷氧四面体的伸缩模振动频率与连接中心磷原子的桥氧数密切相关,随桥氧数增加而升高。此外还解释了模拟图谱与实验谱差异的原因。     

[M(Im)2X2]型配合物的Far-IR和Raman光谱的理论研究

本文用从头计算RHF和密度泛函B3LYP方法以及LanL2DZ，SDD和6-31G(d)基组计算了配合物M(Im)₂X₂ (Im=imidazole；M=Zn(Ⅱ)，Pd(Ⅱ)，Pt(Ⅱ)；X=F，Cl，Br，I)的几何构型以及Far-IR和Raman振动频率。计算结果表明，对Zn(Ⅱ)配合物而言,B3LYP/6-31G(d)方法得到的几何参数与实验值吻合得最好，B3LYP/SDD次之。在计算Far-IR和Raman振动频率时，发现采用6-31G(d)基组，两种方法计算的结果差别不大。对LanL2DZ和SDD基组而言，对计算结果影响较大的是理论方法，基组影响甚微，个别的振动频率基组影响较大，相比较而言，SDD基组得到的结果更好一些。本文所使用的两种计算方法都能得到与实验值比较吻合的结果，而用从头计算RHF方法计算的结果与实验值更接近一些。在此基础上，预测了Pd(Ⅱ)和Pt(Ⅱ)配合物的Far-IR和Raman振动频率。     

Comparative Study of the Structure of Rhodanine, Isorhodanine, Thiazolidine-2,4-dione, and Thiorhodanine

Ab initio (HF and MP2 level) and semiempirical (AM1, PM3, MNDO) calculations on the relative stabilities and structures of the potential tautomeric forms of rhodanine, isorhodanine, thiazolidine-2,4-dione, and thiorhodanine are reported. Ab initio calculations predict that the thiooxo, oxothio, dioxo, and dithio tautomers are the most stable. These results correspond to the known experimental data. Infrared spectra of the investigated compounds were recorded for the region 4000-150 cm^-1, and the characteristic bands were compared with ab initio calculated frequencies at the HF/3-21G^(*)* level.     

Spectra and Structures of Silicon-Containing Compounds. XXIV.* Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,Barriers to Internal Rotation,and Ab Initio Calculations of Ethylsilane

The infrared (3200 to 400 cm^–1) and Raman (3200 to 20 cm^–1) spectra of gaseous and solid ethylsilane, CH₃CH₂SiH₃, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with quantitative depolarization values. The SiH₃ torsional mode has been observed as sum and difference bands with the silicon-hydrogen stretching vibration. Utilizing the torsional fundamental frequency of 132 cm^–1 the threefold periodic barrier of 590 cm^–1 (7.06 kJ/mol) has been obtained. Utilizing the frequencies of the silicon-hydrogen stretches, Si-H bond distances of 1.485 and 1.484 Å have been obtained for the bonds gauche and trans to the methyl group, respectively. Using previously reported rotational constants from seven different isotopomers, the r ₀ parameters have been calculated and are compared to the corresponding r _s parameters. A complete vibrational assignment is proposed that is consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities as well as the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31G(d), 6-311 + G(d,p), and 6-311+G(2d,2p) basis sets at levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and the theoretical values are compared to the experimental values when appropriate.     

Spectra and Structure of Silicon-Containing Compounds. XXV. Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,and Ab Initio Calculations of Ethyl Chlorosilane-Si-d2

The infrared (3200 to 400 cm^–1) spectra of gaseous and solid and Raman (3200 to 20 cm^–1) spectra of liquid and solid ethyl chlorosilane-Si-d₂, CH₃CH₂SiD₂Cl, have been recorded. Both the gauche and trans conformers have been identified in the fluid phases, but only the gauche conformer remains in the solid phase. Variable temperature (–105 to –150°C) studies of the infrared spectra of CH₃CH₂SiH₂Cl dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 78±11 cm^–1 (0.93±0.13 kJ/mol), with the gauche conformer the more stable form. Utilizing the frequencies of the silicon-hydrogen stretches, from the chlorosilane-Si-d isotopomer, Si—H bond distances of 1.481 and 1.480 Å have been obtained for the gauche conformer and 1.481 Å for the trans conformer. Complete vibrational assignments are proposed for both isotopomers which are consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities and the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31(d), 6-311++G(d,p), and 6-311+G(2d,2p) basis sets with full electron correlation by the Moller–Plesset (MP) perturbation method to second order. Continuing the previously reported rotational constants from five different isotopomers and the ab initio predicted structural parameters, adjusted r ₀ parameters have been calculated, which are compared to the corresponding r _s parameters. The results are discussed and the theoretical values are compared to the experimental values when appropriate.Taken in part from the dissertation of Y. E. Nashed, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

Structural features of muscimol, a potent GABAA receptor agonist, crystal structure and quantum chemicalab initio calculations

Muscimol, a constituent of the mushroomAmanita muscaria, is a semirigid analogue of the inhibitory neurotransmitter 4-aminobutyric acid (GABA). X-ray structure determinations and quantum chemicalab initio calculations (HF/6-31G^*) have been carried out on the muscimol zwitterion. The solid-state conformations of the muscimol zwitterion are calculated to be 1.6–2.2 kcal/mol higher in energy than that of the calculated minimum energy structurein vacuo. A comparison of the calculated and experimental structures indicates that the hydrogen bonding network in the solid state significantly affects the geometry of the molecular structure. This conclusion is supported by results ofab initio calculations on binary complexes between muscimol and an ammonium ion and between muscimol and a methoxide anion, simulating observed hydrogen bonding in the crystal structures.     

Conformational stability of chlorocyclohexane from temperature-dependent FT-IR spectra of xenon solutions, r 0 structural parameters, and vibrational assignment

Variable temperature (?55 to ?105 °C) studies of the infrared spectra (4000–400 cm^?1) of chlorocyclohexane (c-C₆H₁₁Cl) dissolved in liquefied xenon have been carried out. The infrared spectra of the gas and solid have also been recorded from 4000–100 cm^?1. By analyzing six conformer pairs in the xenon solution, a standard enthalpy difference of 132 ± 13 cm^?1 (1.58 ± 0.16 kJ/mol) was obtained with the equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 34 ± 1%. The potential surface describing the conformational interchange has been determined and the Fourier coefficients were obtained. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 161 ± 18 cm^?1 from the four largest basis set calculations, which is consistent with the experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 274 ± 15 cm^?1 which is certainly too large. By utilizing the previously reported microwave rotational constants for two isotopomers (³⁵Cl, ³⁷Cl) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r ₀ structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in Å are: r ₀(C₁–C₇,₈) = 1.532(3); r ₀(C₇,₈–C₁₃,₁₄) = 1.536(3); r ₀(C₄–C₁₃,₁₄) = 1.524(3); and r ₀(C₄–Cl₆) = 1.802(5) and the angles in degrees: ∠C₁C₇,₈C₁₃,₁₄ = 111.3(5)º; ∠Cl₆C₄C₁₃,₁₄ = 109.7(5)º with the two dihedral angles ∠C₈C₁C₇C₁₃ = 56.3(10)º and ∠C₁₄C₄C₁₃C₇ = 56.7(10)º. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

On the Nature of Fluoride Ion Hydration

Hydration of aqueous fluoride ions has been studied by theoretical ab initiocalculations in an attempt to understand the experimental Raman spectrum.Calculations for hydrated fluoride, F^– (H₂O)_n where n = 1–10, have been performedat the RHF/6-31 + G^* level. A relatively stable geometry exists for n = 6; abovethis number, additional waters hydrogen bond to water of the hydrated fluoride.On the long time scale of the ab initio calculation or experimental diffractionstudies, the average coordination of fluoride is 6. However, it has been possibleto interpret the low-frequency Raman spectrum on the basis of a singlehydrogen-bonded water molecule, F^– ... HOH. To rationalize these results, it is proposedthat the average coordination of fluoride is 6, but on the time scale of the Ramanexperiment the fluoride is symmetrically bonded to only one hydrogen of onewater molecule.Chairman and Organizer of the Symposium dedicated to Donald Irish. Unfortunately Murray died during the preparation of this special issue     

Crystal Structure and Quantum Chemical Ab Initio Calculations of Ibotenic Acid,an Excitatory Amino Acid Receptor Agonist

Ibotenic acid, a constituent of Amanita muscaria, is a potent NMDA receptor agonist. The structure of the ibotenic acid zwitterion monohydrate in the crystalline state has been determined by X-ray crystallography. The crystal structure has two ibotenic acid monohydrate formula units in the asymmetric unit and features an extensive hydrogen bond network. The ibotenic acid zwitter-ions in the crystalline state are compared to the in vacuo structure calculated by quantum chemical ab initio calculations at the HF/6-31+G* level and the effects of the hydrogen bond network on the structures in the solid state are discussed. The calculated potential energy curve with respect to side-chain orientation displays a single energy minimum. The conformations corresponding to the solid-state conformations are calculated to be ca. 2 kcal/mol higher in energy than the minimum-energy conformation in vacuo.