Matrix isolation and low temperature solid state FTIR spectroscopic study of alpha-furil

Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C(4)H(3)O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol(-1) higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol(-1). These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of alpha-furil were also found to be present in CCl(4) solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, alpha-furil exists uniquely in its most stable conformational state, I.     

Conformational study of monomeric 2,3-butanediols by matrix-isolation infrared spectroscopy and DFT calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).     

Unusual photochemical C-N bond cleavage in the novel methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate

The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions.     

Matrix-isolated monomeric tryptophan: electrostatic interactions as nontrivial factors stabilizing conformers

An extensive analysis of the conformational space of tryptophan (Trp) was performed at the B3LYP/6-311++G(d,p) level and verified by comparison with the infrared spectra of the compound isolated in low-temperature argon and xenon matrixes. Different types of conformers have been unequivocally identified in the matrixes. Type I exhibits the trans arrangement of the carboxylic group and is stabilized by an O-H...N intramolecular H-bond. Types II and III have the carboxylic group in the cis conformation and feature N-H...O=C and N-H...O-C hydrogen bonds, respectively. Three individual conformers of type I were identified in the matrixes. Other conformational degrees of freedom are related with the Calpha-Cbeta-Cgamma=C and C1-Calpha-Cbeta-Cgamma angles (chi1 and chi2, respectively). In proteins, these two dihedral angles define the conformations of the amino acid residues. In monomeric Trp, chi1 adopts the "+" (ca. +90 degrees ) and "-" (ca. -90 degrees ) orientations, while average values of -67.4, 170.5, and 67.6 degrees ("a", "b", and "c", respectively) were found for chi2. Theoretical analysis revealed two important factors in stabilizing the structures of the Trp conformers: the H-bond type and electrostatic interactions. Classified by the H-bond type, the most stable are forms I, followed by II and III. Out of possible combinations of the chi1 and chi2 dihedral angles, "a+", "b+", and "c-" were theoretically found more stable than their "a-", "b-", and "c+" counterparts. Thus, the stabilizing effect of interactions involving the pyrrole ring (which are possible in Ia+, Ib+, and Ic- conformers) is considerably higher compared to those in which the phenyl ring is engaged (existing in the Ia-, Ib-, and Ic+ forms).     

Molecular structure, infrared spectrum, and photochemistry of squaric acid dimethyl ester in solid argon

Squaric acid dimethyl ester (C(6)O(4)H(6); 3,4-dimethoxycyclobut-3-ene-1,2-dione; DCD) was studied by matrix isolation infrared spectroscopy and by density functional theory (B3LYP) and ab initio (MP2) calculations with the 6-31++G(d,p) and 6-311++G(d,p) basis sets. Three conformers of the compound were theoretically predicted. The two most stable conformers were identified in low-temperature argon matrixes and the energy gap between them was determined. The trans-trans conformer (C(2)(v)) was found to be more stable than the cis-trans form (C(s)) by 4.2 kJ mol(-1), in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ mol(-1)). In situ broadband UV irradiation (lambda > 337 nm) of the matrix-isolated compound was found to induce the ring-opening reaction leading to production of the bisketene, 2,3-dimethoxybuta-1,3-diene-1,4-dione as well as the trans-trans --> cis-trans conformational isomerization. The latter phototransformation allowed separation of the infrared spectra of the two conformers initially trapped into a low-temperature matrix. Upon higher energy irradiation (lambda > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl ester (C(5)O(3)H(6); 2,3-dimethoxycycloprop-2-en-1-one), the latter being obtained in two different conformations (trans-trans and cis-trans). According to the experimental data, deltic acid dimethyl ester is produced by decarbonylation of the initially formed bisketene and not by direct CO extrusion from DCD.     

Fluoroformyl trifluoroacetyl disulfide, FC(O)SSC(O)CF3: synthesis, structure in solid and gaseous states, and conformational properties

Fluoroformyl trifluoroacetyl disulfide, FC(O)SSC(O)CF3, is prepared by quantitative reaction between FC(O)SCl and CF(3)C(O)SH. The conformational properties and geometric structure of the gaseous molecule have been studied by vibrational spectroscopy (IR(gas), Raman(liquid), IR(matrix)), gas electron diffraction (GED), and quantum chemical calculations (B3LYP and MP2 methods). The disulfide bond length derived from the GED analysis amounts 2.023(3) Angstroms, and the dihedral angle around this bond, phi(CS-SC), is 77.7(21) degrees, being the smallest dihedral angle measured for noncyclic disulfides in the gas phase. The compound exhibits a conformational equilibrium at room temperature having the most stable form C(1) symmetry with a synperiplanar (sp-sp) orientation of both carbonyl groups with respect to the disulfide bond. A second form was observed in IR spectra of the Ar matrix isolated compound at cryogenic temperatures, corresponding to a conformer that possess the carbonyl bond of the FC(O) moiety in antiperiplanar position with respect to the S-S single bond (ap-sp). A DeltaH degrees = - = 1.34(11) kcal/mol has been determined by IR(matrix) spectroscopy. The structure of single crystal of FC(O)SSC(O)CF3 was determinate by X-ray diffraction analysis at low temperature using a miniature zone melting procedure. The crystalline solid (monoclinic, P2(1)/n, a = 5.240(4)Angstroms, b = 23.319(17)Angstroms, c = 6.196(4)Angstroms, beta = 113.14(3) degrees) consists exclusively of the (sp-sp) conformation. The geometrical parameters agree with those obtained for the molecule in the gas phase.     

Twisted imide bond in noncyclic imides. Synthesis and structural and vibrational properties of N,N-bis(furan-2-carbonyl)-4-chloroaniline

A novel imide compound (C(16)H(10)ClNO(4)) was synthesized in a single step by the reaction of 2-furoic acid with 4-chloroaniline in a 2:1 molar ratio using carbonyldiimidazole (CDI) in dry THF. The structure was supported by spectroscopic and elemental analyses and the single-crystal X-ray diffraction data. Crystallographic studies revealed that the compound crystallized in a monoclinic system with space group P2(1)/c and unit cell dimensions a = 12.2575(5) ?, b = 7.7596(2) ?, c = 15.0234(7) ?, α = γ = 90°, β = 92.771(4)°, V = 1427.25(10) ?(3), Z = 4. The imide bond is twisted, and the O═C-N-C(O) units deviate significantly from planarity with dihedral angles around the imide group reaching ca. -150.3° (C1-N1-C2-O21 = -148.8° and C2-N1-C1-O11 = -151.9°). The nonplanarity of the imide moiety and the related conformational properties are discussed in a combined approach that includes the analysis of the vibrational spectra together with theoretical calculation methods, especially in terms of natural bond orbital (NBO) calculations.     

Conformational behavior of dimethyl 5-methyl-1H,3H-pyrrolo[1,2-c][1,3]thiazole-6,7-dicarboxylate 2,2-dioxide isolated in low-temperature matrixes

The structure of dimethyl 5-methyl-1H,3H-pyrrolo[1,2-c][1,3]thiazole-6,7-dicarboxylate 2,2-dioxide (PTD) was investigated in low-temperature noble gas matrixes (Ar, Kr, Xe), amorphous solid, and the crystalline state by infrared spectroscopy and computational methods. The geometry of PTD conformers is defined by the orientation of two methyl ester groups, which may adopt pseudo-trans or pseudo-cis positions in relation to the pyrrolo-thiazole system. For both methyl ester groups, the latter arrangement was predicted by the calculations to be energetically the most favorable in the isolated molecule. The envelope form of the thiazolidine ring is present in all conformers, with the sulfur atom placed in the apex position, while the pyrrole ring is almost planar. Three types of conformers differing in the orientation of the methyl ester groups relative to the pyrrolo-thiazole system (cis/cis, trans/cis, cis/trans) were identified in the matrixes. The cis/cis forms were found to be the most stable ones in both gaseous state and argon matrixes. On the other hand, the more polar trans/cis forms were found to be stabilized in the more polarizable krypton and xenon matrixes as well as in the neat amorphous and crystalline phases. On the basis of annealing experiments, performed in argon and xenon matrixes up to 35 and 68 K, respectively, conformational changes preceding the aggregation of the compound are suggested.     

Trifluoromethanesulfenyl acetate, CF3S-OC(O)CH3, and trifluoromethanesulfenyl trifluoroacetate, CF3S-OC(O)CF3: unexpected conformational properties

Structural and conformational properties of two sulfenyl derivatives, trifluoromethanesulfenyl acetate, CF3S-OC(O)CH3 (1), and trifluoromethanesulfenyl trifluoroacetate, CF3S-OC(O)CF3 (2), were determined by gas electron diffraction, vibrational spectroscopy, in particular with IR (matrix) spectroscopy, which includes photochemical studies, and by quantum chemical calculations. Both compounds exist in the gas phase as a mixture of two conformers, with the prevailing component possessing a gauche structure around the S-O bond. The minor form, 15(5)% in 1 and 11(5)% in 2 according to IR(matrix) spectra, possesses an unexpected trans structure around the S-O bond. The C=O bond of the acetyl group is oriented syn with respect to the S-O bond in both conformers. UV-visible broad band irradiation of 1 and 2 isolated in inert gas matrixes causes various changes to occur. Conformational randomization clearly takes place in 2 with simultaneous formation of CF3SCF3. For 1 the only reaction channel detected leads to the formation of CH3SCF3 with the consequent extrusion of CO2. Quantum chemical calculations (B3LYP/6-31G and MP2 with 6-31G and 6-311G(2df,pd) basis sets) confirm the existence of a stable trans conformer. The calculations reproduce the conformational properties for both compounds qualitatively correct with the exception of the B3LYP method for compound 2 which predicts the trans form to be prevailing, in contrast to the experiment.     

Conformational structure of gaseous 3-chloropropanoyl chloride by electron diffraction, normal coordinate analysis, and ab initio molecular orbital, and density functional theory calculations

The molecular and conformational structures of 3-chloropropanoyl chloride (CH(2)Cl-CH(2)-C(=O)Cl) have been studied by using gas-phase electron diffraction (GED) data obtained at 22 degrees C (295 K) and ab initio molecular orbital (MO) and density functional theory (DFT) calculations up to the levels of MP4(SDQ) and B3LYP using larger basis sets. Normal coordinate calculations (NCA) taking into account nonlinear vibrational effects were also used in the analyses. The title compound may have up to four low-energy conformers in the gas phase, labeled according to the position of each of the two chlorine atoms in relation to the CCC propanoyl backbone, labeling the carbonyl chlorine torsion angle first: AA, AG, GG, and GA; where A is anti (ideal C-C-C-Cl torsion angle of approximately 180 degrees) and G is gauche (ideal C-C-C-Cl torsion angle of approximately 60 degrees). It has been judged from the experimental GED data and the theoretical calculations, as well as from previously published infrared (IR) studies on the molecule in both the liquid phase and in argon-trapped matrices at 10 K, that the gas phase consists of a mixture of at least three conformers: AA (most stable), AG, and GG, with the possibility of a smaller contribution (<10%) from the higher-energy GA form. The GA conformer cannot be ruled out by the GED experimental data. Relevant structural parameter values obtained from the GED least-squares refinements, with calculated ab initio MO MP2/6-31+G(2d,p) values used as constraints, were as follows (AA values with estimated 2sigma uncertainties): Bond lengths (r(h1)): r(C-C(=O)) = 1.505(4) A, r(C-CH(2)Cl) = 1.520(4) A, r(C=O) = 1.197(4) A, r(C(=O)-Cl) = 1.789(3) A, and r(C-Cl) = 1.782(3) A. Bond angles (angle(h1)): angle CCC = 111.5(11) degrees , angle CCO = 127.0(5) degrees, angle CC(O)Cl = 112.5(3) degrees, and angle CCCl = 110.3(3) degrees. Torsion angles (phi(C-C) = phi(ClCCC)): for AA, phi(1)(C-C(O)) = phi(2)(C-CH(2)Cl) = 180 degrees (assumed for true C(s) symmetry); for AG, phi(1)(C-C(O)) = -140(5) degrees, phi(2)(C-CH(2)Cl) = 76(13) degrees; for GG, phi(1)(C-C(O)) = 46(8) degrees, phi(2)(C-CH(2)Cl) = 77(14) degrees; for GA, phi(1)(C-C(O)) = 67.9 degrees (assumed), phi(2)(C-CH(2)Cl) = 177.8 degrees (assumed). The non-AA conformers all have chiral C(1) symmetry with twice the statistical weight (multiplicity) of C(s). The MP2/6-31+G(2d,p) calculated composition (%) based on the zero-point energy (ZPE) corrected energy differences, and the statistical weights for conformers: AA/AG/GG/GA = 28/35/28/9 was assumed in the final GED refinement. The more recent literature concerning the title molecule, as well as for several related molecules, has been examined and a survey has been attempted in the present article. The new experimental results for 3-chloropropanoyl chloride are discussed and compared with the previously published findings.     

Synthesis, spectroscopic characterization, and conformational properties of trichloromethanesulfenyl acetate, CCl3SOC(O)CH3

Trichloromethanesulfenyl acetate, CCl 3SOC(O)CH 3, belongs to the family of sulfenic esters. This molecule has been characterized by vibrational spectroscopy. The conformational and geometrical properties of this species have been determined by IR and Raman spectroscopy, X-ray diffraction, and quantum chemical calculations. Geometry optimizations of the most stable forms were performed with ab initio (HF, MP2) and density functional theory (B3LYP) methods. According to our data, this compound results in a gauche-syn conformer with C 1 symmetry (gauche orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the most stable geometry, and trans-syn conformer with C s symmetry (trans orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the second stable conformer (1.1 and 0.53 kcal/mol higher in energy than the most stable C 1 form according to the matrix FTIR spectroscopy and MP2/6-31G* level of the theory, respectively). The crystalline solid (monoclinic, P2 1/ n, a = 8.0152(17) A, b = 5.7922(13) A, c = 17.429(4) A, alpha = gamma = 90 degrees , beta = 100.341(3) degrees ) consists exclusively of the main form. The geometrical parameters (X-ray diffraction) are d C-Cl = 1.767(19) A, d C-S = 1.797(2) A, d S-O = 1.663(14) A, d CO = 1.189(2) A, d O-C = 1.389(3) A, d C-C = 1.483(3) A, angles Cl-C-Cl = 110.3(11) degrees , Cl-C-S = 111.8(12) degrees , C-S-O = 97.4(8) degrees , S-O-C = 116.7(11) degrees , O-CO = 122.8(19) degrees , OC-C = 127.1(2) degrees , and the main torsion angles are delta(CSOC) = 105.9(15) degrees and delta(SOC(O)) = 7.6(3) degrees . The geometrical data calculated with B3LYP/6-31G++(3df,3pd), B3LYP/6-311G++(3df,3pd), B3LYP/aug-cc-pVTZ, and MP2/6-31G* are in good agreement with diffraction data.     

Conformation-changing aggregation in hydroxyacetone: a combined low-temperature FTIR, jet, and crystallographic study

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dimers and oligomers. The O-H···O═C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H···O-H hydrogen bonds in competition with isolated O-H···O═C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from ~0° in the gas phase to ~180° in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.     

Structure of Isolated 1,4-Butanediol: Combination of MP2 Calculations, NBO Analysis, and Matrix-Isolation Infrared Spectroscopy

Theoretical calculations at the MP2 level, NBO and AIM analysis, and matrix-isolation infrared spectroscopy have been used to investigate the structure of the isolated molecule of 1,4-butanediol (1,4-BDO). Sixty-five structures were found to be minima on the potential energy surface, and the three most stable forms are characterized by a folded backbone conformation leading to the formation of an intramolecular H-bond. To better characterize the intramolecular interactions and particularly the hydrogen bonds, natural bond orbital analysis (NBO) was performed for the four most stable conformers, and was further complemented with an atoms-in-molecules (AIM) topological analysis. Infrared spectra of 1,4-BDO isolated in low-temperature argon and xenon matrixes show a good agreement with a population-weighted mean theoretical spectrum, and the spectral features of the conformers expected to be trapped in the matrixes were observed experimentally. Annealing the xenon matrix from 20 to 60 K resulted in significant spectral changes, which were interpreted based on the barriers to intramolecular rotation. An estimation of the intramolecular hydrogen bond energy was carried out following three different methodologies.     

Photoisomerization and photochemistry of matrix-isolated 3-furaldehyde

3-Furaldehyde (3FA) was isolated in an argon matrix at 12 K and studied using FTIR spectroscopy and quantum chemistry. The molecule has two conformers, with trans and cis orientation of the O=C-C=C dihedral angle. At the B3LYP/6-311++G(d,p) level of theory, the trans form was computed to be ca. 4 kJ mol(-1) more stable than the cis form. The relative stability of the two conformers was explained using the natural bond orbital (NBO) method. In fair agreement with their calculated relative energies and the high barrier of rotamerization (ca. 34 kJ mol(-1) from trans to cis), the trans and cis conformers were trapped in an argon matrix from the compound room temperature gas phase in proportion ~7:1. The experimentally observed vibrational signatures of the two forms are in a good agreement with the theoretically calculated spectra. Broad-band UV-irradiation (λ > 234 nm) of the matrix-isolated compound resulted in partial trans → cis isomerization, which ended at a photostationary state with the trans/cis ratio being ca. 1.85:1. This result was interpreted based on results of time-dependent DFT calculations. Irradiation at higher energies (λ > 200 nm) led to decarbonylation of the compound, yielding furan, cyclopropene-3-carbaldehyde, and two C(3)H(4) isomers: cyclopropene and propadiene.     

HXeOBr in a xenon matrix

We report on a new noble-gas molecule HXeOBr prepared in a low-temperature xenon matrix from the HBr and N(2)O precursors by UV photolysis and thermal annealing. This molecule is assigned with the help of deuteration experiments and ab initio calculations including anharmonic methods. The H-Xe stretching frequency of HXeOBr is observed at 1634 cm(-1), which is larger by 56 cm(-1) than the frequency of HXeOH identified previously. The experiments show a higher thermal stability of HXeOBr molecules in a xenon matrix compared to HXeOH.     

Importance of entropy in the conformational equilibrium of phenylalanine: a matrix-isolation infrared spectroscopy and density functional theory study

The conformational behavior and infrared spectrum of l-phenylalanine were studied by matrix-isolation infrared spectroscopy and DFT [B3LYP/6-311++G(d,p)] calculations. The fourteen most stable structures were predicted to differ in energy by less than 10 kJ mol(-1), eight of them with abundances higher than 5% at the temperature of evaporation of the compound (423 K). Experimental results suggest that six conformers contribute to the spectrum of the isolated compound, whereas two conformers (IIb(3) and IIIb(3)) relax in matrix to a more stable form (IIb(2)) due to low energy barriers for conformational isomerization (conformational cooling). The two lowest-energy conformers (Ib(1), Ia) differ only in the arrangement of the amino acid group relative to the phenyl ring; they exhibit a relatively strong stabilizing intramolecular hydrogen bond of the O-H...N type and the carboxylic group in the trans configuration (O=C-O-H dihedral angle ca. 180 degrees ). Type II conformers have a weaker H-bond of the N-H...O=C type, but they bear the more favorable cis arrangement of the carboxylic group. Being considerably more flexible, type II conformers are stabilized by entropy and the relative abundances of two conformers of this type (IIb(2) and IIc(1)) are shown to significantly increase with temperature due to entropic stabilization. At 423 K, these conformers are found to be the first and third most abundant species present in the conformational equilibrium, with relative populations of ca. 15% each, whereas their populations could be expected to be only ca. 5% if entropy effects were not taken into consideration. Indeed, phenylalanine can be considered a notable example of a molecule where entropy plays an essential role in determining the relative abundance of the possible low-energy conformational states and then, the thermodynamics of the compound, even at moderate temperatures. Upon UV irradiation (lambda > 235 nm) of the matrix-isolated compound, unimolecular photodecomposition of phenylalanine is observed with production of CO(2) and phenethylamine.     

Preparation and properties of methoxycarbonylsulfenyl isocyanate, CH(3)OC(O)SNCO

Pure methoxycarbonylsulfenyl isocyanate, CH3OC(O)SNCO, is quantitatively prepared by the metathesis reaction between CH3OC(O)SCl and AgNCO. This novel species has been obtained in its pure form and characterized by 1H and 13C NMR, UV-vis, FTIR, and FT-Raman spectroscopy. The conformational properties of the gaseous molecule have been studied by vibrational spectroscopy and quantum chemical calculations (B3LYP and MP2 methods). The compound exhibits a conformational equilibrium at room temperature having the most stable form CS symmetry with the C=O double bond synperiplanar with respect to the S-N single bond. A second form was observed in the IR spectrum and corresponds to a conformer possessing the C-S bond antiperiplanar with respect to the N=C double bond of the isocyanate group. The structure of a single crystal of CH3OC(O)SNCO was determined by X-ray diffraction analysis at low temperature using a miniature zone melting procedure. The crystalline solid (triclinic, P1, a = 8.292(6) A, b = 9.839(7) A, c = 11.865(8) A, alpha = 67.290(2) degrees , beta = 71.5570(10) degrees , gamma = 83.4850(10) degrees and Z = 6) shows the presence of molecules having exclusively a synperiplanar conformation with respect to the three phi(CO-C=O), phi(O=C-SN), and phi(CS-N=C) dihedral angles.     

S-(fluoroformyl)O-(trifluoroacetyl) thioperoxide, FC(O)S-OC(O)CF3: gas-phase structure and conformational properties

The geometric structure and conformational properties of S-(fluoroformyl)O-(trifluoroacetyl) thioperoxide, FC(O)S-OC(O)CF3, were investigated by gas electron diffraction, matrix isolation infrared spectroscopy, and quantum chemical calculations (B3LYP with the 6-31G and aug-cc-pVTZ basis sets and MP2 with the 6-31G basis set). The experimental methods result in a mixture of two conformers with gauche conformation around the S-O bond. In the main conformer (82(7)% according to GED at 298 K), the C=O bond of the FC(O) group is oriented syn with respect to the S-O bond and phi(C-S-O-C) = 75(3) degrees . In the minor conformer (18(7)%), this C=O is oriented anti. Both conformers possess syn orientation of the C=O bond of the CF3C(O) group. The conformational properties and geometric parameters are reproduced reasonably well by the quantum chemical calculations, except for the S-O bond length, which is predicted too long by 0.04 A (B3LYP/aug-cc-pVTZ).     

Effect of torsional potential on the predicted phase behavior of n-alkanes

The effect of torsional potential on the predictions of simulation for vapor–liquid equilibria of n-alkanes is determined. Calculations are performed with histogram-reweighting Monte Carlo simulations in the grand canonical ensemble. Decreasing the magnitude of energy barriers to dihedral rotation or allowing free rotation is found to have no effect on the predicted vapor–liquid equilibria. Restriction of the dihedral angles to a Gaussian distribution around the minimum energy conformation causes an under-prediction of the liquid densities and critical temperatures by a maximum of 7% and 2%, respectively, with discrepancies increasing monotonically with the number of dihedral angles present in a molecule. No significant deviation in vapor pressure is observed for any compound, regardless of torsional potential used. An analysis of the conformational behavior reveals that restriction of the dihedral sampling has a measurable effect on excluded volume of the molecule, and this change of conformational behavior is responsible for the reduction in the predicted saturated liquid densities observed in this work. Similar calculations for force fields employing reduced dihedral potentials or freely jointed chains show little change in the predicted excluded volume compared to the reference force field.     

Trifluoromethyl chlorosulfonate, CF3OSO2Cl: gas phase and crystal structure, conformation and vibrational analysis studied by experimental and theoretical methods

The geometric structure and conformational properties of trifluoromethyl chlorosulfonate (chlorosulfuric acid trifluoromethyl ester), CF(3)OSO(2)Cl, have been determined by X-ray crystallography, gas electron diffraction (GED), and vibrational spectroscopy (IR(gas), IR(matrix), and Raman(liquid)). These experimental investigations were supplemented by quantum chemical calculations (B3LYP with 6-311G* and 6-311+G(3df) basis sets). All experimental methods result in a single conformer with gauche orientation of the CF(3) group relative to the S[bond]Cl. The dihedral angle delta(COSCl) is determined to be 91.7(3) degrees in the crystal and 94(3) degrees in the gas phase. This dihedral angle corresponds to a near-eclipsed orientation of the O[bond]C relative to one of the S[double bond]O bonds (delta(CO[bond]SO) = -23.0(3) degrees and -21(3) degrees in the crystal and gas phase, respectively).