Vibrational spectroscopy of cis- and trans-formic acid in solid argon

Absorption spectra of cis and trans conformers of formic acid (HCOOH) isolated in solid argon are analyzed in the mid-infrared (4000-) and near-infrared (7800-) regions. The HCOOH absorption spectrum reveals matrix-site splitting for the trapped molecule. Narrowband tunable infrared radiation is used to pump a suitable vibrational transition of the trans conformer in order to promote site-selectively the conversion to the cis conformer and separate the spectral features of each site group. Several anharmonic resonances are identified for both conformers. The results of anharmonic vibrational ab initio calculations (CC-VSCF) for the trans and cis conformers of formic acid are reported and compared with the experimental spectra.     

Gas phase A-band short time photodissociation dynamics of trans and gauche conformations of 1-iodopropane and comparison with the solution phase short time photodissociation dynamics

A-band resonance Raman spectra are reported for gas phase 1-iodopropane. The gas phase absorption spectrum and resonance Raman intensities were simulated using time-dependent wavepacket calculations and a simple model in order to extract the A-band short time photo-dissociation dynamics for the trans and gauche conformers of 1-iodopropane. The gas phase short time dynamics for trans and gauche are very similar to the results obtained from a reanalysis of corresponding solution phase spectra. This indicates that solvation has little effect on the A-band short time photodissociation dynamics. However, the electronic dephasing parameters for the gauche conformer increase significantly upon solvation while the trans conformer parameters are almost the same in the gas and solution phases. This suggests that the gauche conformer in the A-band excited electronic state undergoes stronger interaction with the solvent than the trans conformer to give rise to faster electronic dephasing upon solvation for the gauche conformer.     

Pyrolysis of amines: Infrared spectrum of 1-aminopropene

The infrared absorption spectrum of 1-aminopropene was observed for the first time in the gas phase. This molecule was produced by the thermal decomposition of cyclopropylamine. Two rotational isomers, cis and trans around the C=C double bond, occur. The relative yield of the conformers depends on the experimental conditions. The main conformer could not be identified experimentally, but an ab initio MO calculation suggests that the trans conformer is a little more stable form than the cis. These empirical assignments of the vibrational modes are supported by the ab initio MO calculation.     

Structural and vibrational study of 2‐(2′‐ furyl)‐4,5‐1H‐dihydroimidazole

In this study 2‐(2′‐furyl)‐4,5‐1H‐dihydroimidazole (1) was prepared and then characterized by infrared, Raman, and multidimensional nuclear magnetic resonance (NMR) spectroscopies. The crystal and molecular structures of 1 were determined by X‐ray diffraction methods. The density functional theory (DFT) and second‐order Møller–Plesset theory (MP2) with Pople's basis set show that there are two conformers for the title molecule that have been theoretically determined in the gas phase, and that only one of them, conformer I, is present in the solid phase. NMR spectra observed for 1 were successfully compared with the calculated chemical shifts at the B3LYP/6‐311++G** level theorized for this conformer. The harmonic vibrational frequencies for the optimized geometry of the latter conformer were calculated at the B3LYP/6‐311++G** level in the approximation of the isolated molecule. For a complete assignment of the IR and Raman spectra in the solid phase of 1 , DFT calculations were combined with Pulay´s scaled quantum mechanics force field (SQMFF) methodology to fit the theoretical frequency values to the experimental ones. Copyright © 2009 John Wiley & Sons, Ltd.     

Substituent effects on the conformational stability of allyl fluorides

By the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCH?CHCH₂F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for Y?H, Cl, Me, Vinyl, CF₃, CN, CHO, and NO₂, whereas the gauche is favored for Y?OMe, OH. A correlation of ΔH with the substituent constants σ⁺(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH₂?CHCH₂F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C₂–C₃ vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C₂–C₃ vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive π_C?C→σ*_{C? F} interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman and ab initio study of the conformational isomerism in the 1‐ethyl‐3‐methyl‐imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid

The calculated and experimental Raman spectra of the (EMI⁺)TFSI⁻ ionic liquid, where EMI⁺ is the 1‐ethyl‐3‐methylimidazolium cation and TFSI⁻ the bis(trifluoromethanesulfonyl)imide anion, have been investigated for a better understanding of the EMI⁺ and TFSI⁻ conformational isomerism as a function of temperature. Characteristic Raman lines of the planar (p) and non‐planar (np) EMI⁺ conformers are identified using the reference (EMI⁺)Br⁻ salt. The anion conformer of C₂ symmetry is confirmed to be more stable than the cis (C₁) one by 4.5 ± 0.2 kJ mol⁻¹. At room temperature, the population of trans (C₂) anions and np cations is 75 ± 2% and 87 ± 4%, respectively. Fast cooling quenches a metastable glassy phase composed of mainly C₂ anion conformers and p cation conformers, whereas slow cooling gives a crystalline phase composed of C₁ anion conformers and of np cation conformers. Copyright © 2006 John Wiley & Sons, Ltd.     

Vibrational spectroscopic studies,conformations and ab initio calculations of 1,2‐bis(trifluorosilyl)ethane (SiF3CH2CH2SiF3)

Infrared spectra of 1,2‐bis(trifluorosilyl)ethane (SiF₃CH₂CH₂SiF₃) were obtained in the vapour and liquid phases, in argon matrices and in the solid phase. Raman spectra of the compound as a liquid were recorded at various temperatures between 293 and 270 K and spectra of an apparently crystalline solid were observed. The spectra revealed the existence of two conformers (anti and gauche) in the vapour, liquid and in the matrix. When the vapour was chock‐frozen on a cold finger at 78 K and annealed to 150 K, certain weak Raman bands vanished in the crystal. The vibrational spectra of the crystal demonstrated mutual exclusion between IR and Raman bands in accordance with C_2h symmetry. Intensity variations between 293 and 270 K of pairs of various Raman bands gave ΔH(gauche—anti) = 5.6 ± 0.5 kJ mol⁻¹ in the liquid, suggesting 85% anti and 15% gauche in equilibrium at room temperature. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices, is the low‐energy conformer in the liquid and is also present in the crystal. The spectra of both conformers have been interpreted, and 34 anti and 17 gauche bands were tentatively identified. Ab initio and density functional theory (DFT) calculations were performed giving optimized geometries, infrared and Raman intensities and anharmonic vibrational frequencies for both conformers. The conformational energy difference derived in CBS‐QB3 and in G3 calculations was 5 kJ mol⁻¹. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopy of trans and cis deuterated formic acid (HCOOD): Anharmonic calculations and experiments in argon and neon matrices

The absorption spectra of trans and cis conformers of deuterated formic acid (HCOOD) isolated in argon and neon matrices are analyzed in the mid-infrared and near-infrared spectral regions (7900-450 cm⁻¹). Vibrational excitation by narrow-band IR radiation is used to convert the lower-energy trans conformer to the higher-energy cis form. A large number of overtone and combination bands are identified. The results of anharmonic vibrational calculations (CC-VSCF) for both conformers are reported and compared to the experimental spectra.     

Reaction products from a discharge of N2 and H2S: The microwave spectrum of NH2SH

Thiohydroxylamine has been identified as one of the reaction products from the discharge reaction of N₂ + H₂S. Both cis and trans conformers have been observed. The rotational spectra have been studied from 56 to 170 GHz for the normal species and several deuterated isotopic species of each conformer. The electric dipole moments of both conformers have been determined. A number of the transitions of the cis conformer exhibit splittings due to the nuclear quadrupole moment of the ¹⁴N nucleus. A least squares fit of the frequency splittings have led to an analysis of the eQq values. Ab initio calculations using a 4-31G basis set both with and without polarization functions have been carried out to aid in the analysis and to provide a final structural comparison with the microwave results.     

Infrared and Raman Spectra,conformations, ab initio calculations and spectral assignments of 1,3‐disilabutane (SiH3CH2SiH2CH3)

Raman spectra of 1,3‐disilabutane (SiH₃CH₂SiH₂CH₃) as a liquid were recorded at 293 K and as a solid at 78 K. In the Raman cryostat at 78 K an amorphous phase was first formed, giving a spectrum similar to that of the liquid. After annealing to 120 K, the sample crystallized and large changes occurred in the spectra since more than 20 bands present in the amorphous solid phase vanished. These spectral changes made it possible to assign Raman bands to the anti or gauche conformers with confidence. Additional Raman spectra were recorded of the liquid at 14 temperatures between 293 and 137 K. Some Raman bands changed their peak heights with temperature but were countered by changes in linewidths, and from three band pairs assigned to the anti and gauche conformers, the conformational enthalpy difference Δ_confH(gauche–anti) was found to be 0 ± 0.3 kJ mol⁻¹ in the liquid. Infrared spectra were obtained in the vapor and in the liquid phases at ambient temperature and in the solid phases at 78 K in the range 4000–400 cm⁻¹. The sample crystallized immediately when deposited on the CsI window at 78 K, and many bands present in the vapor and liquid disappeared. Additional infrared spectra in argon matrixes at 5 K were recorded before and after annealing to temperatures 20–34 K. Quantum chemical calculations were carried out at the HF, MP2 and B3LYP levels with a variety of basis sets. The HF and DFT calculations suggested the anti conformer as the more stable one by ca 1 kJ mol⁻¹, while the MP2 results favored gauche by up to 0.4 kJ mol⁻¹. The Complete Basis Set method CBS‐QB3 gave an energy difference of 0.1 kJ mol⁻¹, with anti as the more stable one. Scaled force fields from B3LYP/cc‐pVQZ calculations gave vibrational wavenumbers and band intensities for the two conformers. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational spectroscopy and DFT calculations of di‐amino acid cyclic peptides. Part I: cyclo(Gly‐Gly), cyclo(L‐Ala‐L‐Ala) and cyclo(L‐Ala‐Gly) in the solid state and in aqueous solution

Investigations of the vibrational spectra of cyclo(Gly‐Gly), cyclo(L‐Ala‐L ‐Ala) and cyclo(L ‐Ala‐Gly) are reported. Raman scattering and Fourier transform infrared (FTIR) spectra of solid‐state and aqueous protonated samples, as well as their corresponding N‐deuterated isotopomers, have been examined. In addition, density functional theory (DFT) (B3‐LYP/cc‐pVDZ) calculations of molecular structures and their associated vibrational modes were carried out. In each case, the calculated structures of lowest energy for the isolated gas‐phase molecules have boat conformations. Assignments have been made for the observed Raman and FTIR vibrational bands of the cyclic di‐amino acid peptides (CDAPs) examined. Raman polarization studies of aqueous phase samples are consistent with C₂ and C₁ symmetries for the six‐membered rings of cyclo(L‐Ala‐L‐Ala) and cyclo(L‐Ala‐Gly), respectively. There is a good correlation between experimental and calculated vibrational bands for the three CDAPs. These data are in keeping with boat conformations for cyclo(L‐Ala‐L‐Ala) and cyclo(L‐Ala‐Gly) molecules, predicted by the ab initio calculations, in both the solid and aqueous solution states. However, Raman spectroscopic results might infer that cyclo(L‐Ala‐Gly) deviates only slightly from planarity in the solid state. The potential energy distributions of the amide I and II modes of a cis‐peptide linkage are shown to be significantly different from those of the trans‐peptides. For example, deuterium shifts have shown that the cis‐amide I vibrations found in cyclo(Gly‐Gly), cyclo(L‐Ala‐L‐Ala), and cyclo(L‐Ala‐Gly) have larger N‐H contributions compared to their trans‐amide counterparts. Compared to trans‐amide II vibrations, cis‐amide II vibrations show a considerable decrease in N H character. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopic studies of the structure of di‐amino acid peptides. Part II: cyclo(L‐Asp‐L‐Asp) in the solid state and in aqueous solution

Solid‐state protonated and N,O‐deuterated Fourier transform infrared (IR) and Raman scattering spectra together with the protonated and deuterated Raman spectra in aqueous solution of the cyclic di‐amino acid peptide cyclo(L ‐Asp‐L ‐Asp) are reported. Vibrational band assignments have been made on the basis of comparisons with previously cited literature values for diketopiperazine (DKP) derivatives and normal coordinate analyses for both the protonated and deuterated species based upon DFT calculations at the B3‐LYP/cc‐pVDZ level of the isolated molecule in the gas phase. The calculated minimum energy structure for cyclo(L ‐Asp‐L ‐Asp), assuming C₂ symmetry, predicts a boat conformation for the DKP ring with both the two L ‐aspartyl side chains being folded slightly above the ring. The CO stretching vibrations have been assigned for the side‐chain carboxylic acid group (e.g. at 1693 and 1670 cm⁻¹ in the Raman spectrum) and the cis amide I bands (e.g. at 1660 cm⁻¹ in the Raman spectrum). The presence of two bands for the carboxylic acid CO stretching modes in the solid‐state Raman spectrum can be accounted for by factor group splitting of the two nonequivalent molecules in a crystallographic unit cell. The cis amide II band is observed at 1489 cm⁻¹ in the solid‐state Raman spectrum, which is in agreement with results for cyclic di‐amino acid peptide molecules examined previously in the solid state, where the DKP ring adopts a boat conformation. Additionally, it also appears that as the molecular mass of the substituent on the C^α atom is increased, the amide II band wavenumber decreases to below 1500 cm⁻¹; this may be a consequence of increased strain on the DKP ring. The cis amide II Raman band is characterized by its relatively small deuterium shift (29 cm⁻¹), which indicates that this band has a smaller N H bending contribution than the trans amide II vibrational band observed for linear peptides. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of hydrogen bonding in the vibrational spectra of trans‐cinnamic acid

A computationally‐assisted methodology to assign the vibrational spectra of molecular materials (PiMM) has been successfully applied to the Raman and infrared (IR) spectra of trans‐cinnamic acid (t‐CIA) in the solid state. The effects of molecular association through O H···O hydrogen‐bonding in both the wavenumber and the intensity of the bands are accurately described by a single dimer structure. This supports the discrimination between the possible conformers in the solid, the complete evaluation of hydrogen‐bonding effects, and the full assignment of the vibrational spectra of t‐CIA. Copyright © 2008 John Wiley & Sons, Ltd.     

Conformational,vibrational, and structural studies of 2,2,3,3,3‐pentafluoropropylamine from Raman and infrared spectra of gas,liquid, xenon solutions,and solid supported by ab initio calculations

The Raman and infrared spectra (3500–50 cm⁻¹) of the gas, liquid or solution, and solid were recorded of 2,2,3,3,3‐pentafluoropropylamine (CF₃CF₂CH₂NH₂) and the ND₂ isotopomer. Variable temperature (−55 to − 100 °C) studies of the infrared spectra (3600–400 cm⁻¹) of samples dissolved in liquid xenon have been carried out. From these data, two of the five possible conformers have been identified and their relative stabilities obtained. The enthalpy difference was determined between the more stable Tt conformer and the less stable Tg form to be 280 ± 14 cm⁻¹ (3.35 ± 0.17 kJ/mol). The first indicator is the NCCC dihedral angle (G = gauche or T = trans), and the second one (g = gauche or t = trans) is the relative position of the lone pair of electrons on nitrogen with respect to the β‐carbon. The percentage of the Tg conformer at ambient temperature is estimated to be 34 ± 2%. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for most of the observed bands for both isotopomers, supported by MP2(full)/6‐31G(d) ab initio calculations to predict the harmonic force fields, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311 + G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2011 John Wiley & Sons, Ltd.     

Conformational stability from Raman spectra,r0 structural parameters,and vibrational assignment of methylcyclohexane

The Raman spectra (3500–50 cm⁻¹) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm⁻¹ in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 °C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair‐equatorial conformer and the chair‐axial form was determined to be 712 ± 71 cm⁻¹ (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm⁻¹ (8.50 kJ/mol) from the MP2(full)/6‐311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6‐31G(d) ab initio calculations. With two scaling factors of 0.88 for the C‐H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B‐type for A″ modes) a complete vibrational assignment has been made for the chair‐equatorial conformer. Predicted r₀ structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.     

Conformational analysis: Part 42. A modelling and LIS/NMR investigation of the conformations of some aromatic SO compounds

A refined Lanthanide‐Induced‐Shift Analysis (LISA), in which both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately, is used with molecular mechanics (MMFF94) and ab initio (RHF/6‐31G, RHF/6‐311G** and B3LYP/6‐311G**) calculations to investigate the conformations of methyl para‐tolyl sulphoxide 1 , sulphone 2 and sulphonate 3 and the conformational equilibria of methyl 2,4‐dimethylphenyl sulphoxide 4 and sulphone 5 . In 1 acceptable agreement factors (AFs) in the LISA analysis were only obtained when the SO/phenyl dihedral angle was increased from that (10–15°) of the computed geometries to 30–40°. The calculated geometries for 2 had the methyl group orthogonal to the phenyl ring and gave good AF in the LISA analysis. In both compounds, the AF were in the order MMFF94 ≤ B3LYP < RHF with the larger basis sets giving better agreement than the limiting 6‐31G* function. In 3 all the geometries again had the methoxy group orthogonal to the phenyl ring but, while the ab initio geometries still gave good AFs, the MMFF94 structure gave an AF which was only just acceptable. The unsymmetrical compounds 4 and 5 display conformational equilibria. In 4 two energy minima were found, both with the SMe group orthogonal to the phenyl ring, with the SO oxygen cis or trans to the 2‐methyl group. The energy difference (cis–trans) was calculated as 1.6 kcal/mol (MMFF94), 2.7 (6‐31G*), 1.9 (6‐311G**) but ?0.11 (DFT). The LISA analyses on the separate conformers gave poor agreement (ca. 3%) for the trans conformer but increasing the SO/Ph dihedral angle again gave an acceptable value (ca. 1.4%). Inclusion of even small amounts of the cis conformer in the LIS analysis gave poorer results. Thus, the LISA analysis confirms the MM and RHF prediction that the trans conformer is more stable and suggests that the conformer energy difference in solution is equal to or greater than the largest calculated value of 2.7 kcal/mol. In 5 all the calculations gave two mirror image conformations with the SMe orthogonal to the phenyl ring. The LISA analysis in this case is complicated by the fact that in the stable conformer the complexing oxygen atoms are not equivalent. This was resolved by considering both coordination sites in one Z‐matrix and varying the % coordination on each oxygen atom. This gave an acceptable AF of ca. 1.0% for all the geometries with ca. 2:1 coordination on the oxygen atoms trans and cis to the C2‐methyl. The difference in the geometries and conformer energies found by LISA for these molecules in solution compared to the theoretical values may be due to interactions with the chloroform solvent for these polar compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural and vibrational analyses of 2‐(2‐benzofuranyl)‐2‐imidazoline

We have studied 2‐(2‐benzofuranyl)‐2‐imidazoline (BFI) and characterized it by using infrared and Raman spectroscopies. The density functional theory (DFT) method together with Pople's basis set shows that two conformers exist for the title molecule as have been theoretically determined in the gas phase and that, probably, an average of both conformations is present in the solid phase. The harmonic vibrational wavenumbers for the optimized geometry of the latter conformer were calculated at the B3LYP/6‐31G* level in the proximity of the isolated molecule. For a complete assignment of the IR and Raman spectra in the compound in the solid phase, DFT calculations were combined with Pulay's scaled quantum mechanics force field (SQMFF) methodology in order to fit the theoretical wavenumbers to the experimental ones. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular structure,vibrational spectroscopic studies and NBO analysis of the 3,5‐dichlorophenylboronic acid molecule by the density functional method

In this study, the Fourier‐transform infrared (FT‐IR) and FT‐Raman spectra of 3,5‐dichlorophenylboronicacid (3,5‐dcpba) were recorded in the solid phase. The structural and spectroscopic analysis of the 3,5‐dichlorophenylboronic was made by using density functional harmonic calculations. There are three conformers for this molecule. The computational results diagnose the most stable conformer of 3,5‐dcpba as the ct form. The geometrical parameters and energies have been obtained for all three conformers from DFT (B3LYP) with 6‐311+ + G(d,p) basis set calculations. The vibrations of stable and unstable conformers of 3,5‐dcpba are researched by using quantum chemical calculations. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes calculated with the scaled quantum mechanics (SQM) method. The stability of the molecule arising from hyperconjugative interactions and charge delocalization has been analyzed using the natural bond orbital (NBO) analysis. The results show that the charge in electron density (ED) in the π* and σ* antibonding orbitals and E2 energies confirms the occurrence of ıntermolecular charge transfer (ICT) within the molecule. Finally, the calculation results were applied to simulated infrared and Raman spectra of the title compound, which show agreement with the observed spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

IR/Raman spectroscopy and DFT calculations of cyclic di‐amino acid peptides. Part III: comparison of solid state and solution structures of cyclo(L‐Ser‐L‐Ser)

B3‐LYP/cc‐pVDZ calculations of the gas‐phase structure and vibrational spectra of the isolated molecule cyclo(L ‐Ser‐L ‐Ser), a cyclic di‐amino acid peptide (CDAP), were carried out by assuming C₂ symmetry. It is predicted that the minimum‐energy structure is a boat conformation for the diketopiperazine (DKP) ring with both L ‐seryl side chains being folded slightly above the ring. An additional structure of higher energy (15.16 kJ mol⁻¹) has been calculated for a DKP ring with a planar geometry, although in this case two fundamental vibrations have been calculated with imaginary wavenumbers. The reported X‐ray crystallographic structure of cyclo(L ‐Ser‐L ‐Ser), shows that the DKP ring displays a near‐planar conformation, with both the two L ‐seryl side chains being folded above the ring. It is hypothesized that the crystal packing forces constrain the DKP ring in a planar conformation and it is probable that the lower energy boat conformation may prevail in the aqueous environment. Raman scattering and Fourier‐transform infrared (FT‐IR) spectra of solid state and aqueous solution samples of cyclo(L ‐Ser‐L ‐Ser) are reported and discussed. Vibrational band assignments have been made on the basis of comparisons with the calculated vibrational spectra and band wavenumber shifts upon deuteration of labile protons. The experimental Raman and IR results for solid‐state samples show characteristic amide I vibrations which are split (Raman: 1661 and 1687 cm⁻¹, IR: 1666 and 1680 cm⁻¹), possibly due to interactions between molecules in a crystallographic unit cell. The cis amide I band is differentiated by its deuterium shift of ∼30 cm⁻¹, which is larger than that previously reported for trans amide I deuterium shifts. A cis amide II mode has been assigned to a Raman band located at 1520 cm⁻¹. The occurrence of this cis amide II mode at a wavenumber above 1500 cm⁻¹ concurs with results of previously examined CDAP molecules with low molecular weight substituents on the C^α atoms, and is also indicative of a relatively unstrained DKP ring. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectra and crystal structure of the di‐amino acid peptide cyclo(L‐Met‐L‐Met): comparison of experimental data and DFT calculations

Experimental Raman and FT‐IR spectra of solid‐state non‐deuterated and N‐deuterated samples of cyclo(L ‐Met‐L ‐Met) are reported and discussed. The Raman and FT‐IR results show characteristic amide I vibrations (Raman: 1649 cm⁻¹, infrared: 1675 cm⁻¹) for molecules exhibiting a cis amide conformation. A Raman band, assigned to the cis amide II vibrational mode, is observed at ∼1493 cm⁻¹ but no IR band is observed in this region. Cyclo(L ‐Met‐L ‐Met) crystallises in the triclinic space group P1 with one molecule per unit cell. The overall shape of the diketopiperazine (DKP) ring displays a (slightly distorted) boat conformation. The crystal packing employs two strong hydrogen bonds, which traverse the entire crystal via translational repeats. B3‐LYP/cc‐pVDZ calculations of the structure of the molecule predict a boat conformation for the DKP ring, in agreement with the experimentally determined X‐ray structure. Copyright © 2009 John Wiley & Sons, Ltd.