Ab initio comprehensive conformational analysis of 2'-deoxyuridine, the biologically significant DNA minor nucleoside, and reconstruction of its low-temperature matrix infrared spectrum

A comprehensive conformational analysis of isolated 2'-deoxyuridine (dU), a minor DNA nucleoside, has been performed by means of ab initio calculations at the MP2/6-311++G (d,p)//DFT B3LYP/6-31G (d,p) level of theory. At 298.15 and 420 K, all 94 allowed conformers of dU are within 8.96 and 7.91 kcal/mol Gibbs energy ranges, respectively. Syn orientation for the base and South (S) conformers for the sugar dominate at 298.15 K: syn/anti=62.3%:37.7% and S/N=77.2%:22.8%. At 420 K in the majority of conformers, the base is anti oriented and the population of North (N) sugars increases: syn/anti=39.3%:60.7% and S/N=63.0%:37.0%. Values of all conformational parameters and correlations between them, as well as their correlations with valence bonds, and also correlations between valence bonds and angles were estimated. In general, 14 types of intramolecular H-bonds were detected (1-3 H-bonds per conformer, the total number 175), namely, C1'H...O2 (16 H-bonds), C2'H1...O5' (9), C2'H2...O2 (21), C3'H...O2 (21), C5'H1...O2 (14), C5'H2...O2 (11), C6H...O4' (37), C6H...O5' (22), C3'H...HC6 (4), O5'H...HC6 (2), O3'H...O5' (5), O5'H...O4' (1), O5'H...O3' (4), and O5'H...O2 (8). Geometric, vibrational, structural-topological, and energetic features of the OH...O intramolecular H-bonds in dU conformers were determined. The close similarity between energetic and geometric characteristics of dU and thymidine DNA-like conformers in anti and relevant syn conformations and their transition states of the anti-->syn interconversion implies that mismatch DNA glycosylase discriminates between the two nucleosides, mainly because of the difference in the shapes of their bases. Convolution of calculated IR spectra of all the dU conformers within the limits 3400-3700 cm(-1) appears to be consistent with its low-temperature matrix IR spectrum (Ivanov et al. Spectrochim. Acta, Part A 2003, 59, 1959), wavenumber discrepancy not exceeding 1%. It was concluded that, for a reliable reproduction of the experimental spectrum, the whole set of conformers should be taken into consideration. The suggested method makes reconstruction of the isolated nucleoside IR spectrum at a physiological interval of temperature reasonably possible.     

Conformational structures and vibrational spectra of isolated pyrimidine nucleosides: Fourier transform infrared matrix isolation study of 2-deoxyuridine

The conformational structures of 2-deoxyuridine (dU) were investigated using Fourier transform infrared (FTIR) matrix isolation spectroscopy. For the first time the FTIR spectra of dU in Ar matrices were obtained in the range 4000-200 cm(-1). The stabilities of conformers were estimated by the methods HF/3-21G (p), HF/6-31G (d,p) and MP2/6-31G (d,p). Ab initio calculations of the infrared spectra were performed by the methods HF/3-21G (p) and HF/6-31G (d,p). The actual occupancy of conformational isomers in matrix samples was determined. It was shown that anti-conformers of dU are dominant. The ribose rings of the main anti-conformers dU _a0, dU _a1 are in the C2'-endo conformation, but the ribose rings of minor anti-conformers dU_a2, dU_a3 have the C3'-endo conformation, stabilized by intramolecular hydrogen bonds O3'H...O5' and O5'H...O3', accordingly. Syn-conformers of dU are stabilized by the intramolecular hydrogen bond O5'H...O2 and the dominant conformation of the ribose ring is C2'-endo.     

How many conformers determine the thymidine low-temperature matrix infrared spectrum? DFT and MP2 quantum chemical study

A comprehensive conformational analysis of isolated 2'-beta-deoxy-thymidine (T), canonical DNA nucleoside, has been performed by means of ab initio calculations at the MP2/6-311++G(d,p)//DFT B3LYP/6-31G(d,p) level of theory. At 298.15 K, all 92 conformers of isolated dT are within a 7.49 kcal/mol Gibbs energy range. Syn orientation for the base and South (S) conformers for the sugar dominate at this temperature: syn/anti = 61.6%:38.4% and S/N = 74.5%:25.5%. However, at 420 K, the majority of conformers contain anti base and the population of North (N) sugars increases: syn/anti = 38.0%:62.0% and S/N = 59.5%:40.5%. The whole conformational parameters (P, chi, gamma, delta, beta, epsilon, nu max) were analyzed as well as the energies of the OH...O intramolecular H-bonds on the basis of nu(OH) stretching vibrations. Convolution of calculated IR spectra of all of the T conformers appears consistent with its low-temperature matrix spectrum (Ivanov et al. Low Temp. Phys. 2003, 29, 809). The maximal discrepancy in frequencies between calculated and experimental spectra is less than 1%. A conclusion was made that for reliable reconstruction of the isolated nucleoside IR spectrum the quasi whole set of conformers should be taken into consideration. In essence, this result opens up a possibility to reconstruct IR spectra of isolated nucleosides at physiological temperatures with rather satisfactory probability.     

Intramolecular hydrogen bonds in canonical 2'-deoxyribonucleotides: an atoms in molecules study

The molecular structure and relative stability of different conformers of isolated canonical 2'-deoxyribonucleotides thymidine-5'-phosphate (pdT), 2-deoxycytidine-5'-phosphate (pdC), 2-deoxyadenosine-5'-phosphate (pdA), and 2'-deoxyguanosine-5'-phosphate (pdG) were calculated using the B3LYP/6-31++G(d,p) level of theory. The results of the calculations reveal that, for all nucleotides except pdG, conformers with a syn orientation of the base do not correspond to a minimum on the potential energy surface. In the case of pdA and pdC, conformers with an orthogonal orientation of the nucleobase are located instead, north/syn conformers. These conformers as well as syn conformers of pdG are stabilized by intramolecular N-H...O hydrogen bonds. Analysis of the electron density distribution within the atoms in molecules theory reveals the presence of numerous C-H...O hydrogen bonds in the nucleotides. However, a more detailed consideration of the properties of these bonds demonstrates that many of them should be considered as strong attractive electrostatic interactions rather than true hydrogen bonds. True hydrogen bonds are represented mainly by C6/ C8-H...O5'/O-P in anti conformers and the N-H...O-P bonds in syn conformers. It is demonstrated that the values of ellipticity of the electron density at the bond critical point (BCP) and the distance between BCP and ring critical point are the most reliable indicators for determining the true intramolecular hydrogen bonds.     

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 according to PCM. Also, the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM simulation (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.     

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).     

Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended pi-conjugated cyclometalated ligands

Reacting K2PtCl4 with the tridentate R-C(wedge)N(wedge)C-H2 ligands 2,6-di-(2'-naphthyl)-4-R-pyridine (R = H, 1a; Ph, 1b; 4-BrC6H4, 1c; 3,5-F2C6H3, 1d) in glacial acetic acid, followed by heating in dimethyl sulfoxide (DMSO), gave complexes [(R-C(wedge)N(wedge)C)Pt(DMSO)] (2a-d). In the crystal structures of 2a-c, the molecules are paired in a head-to-tail orientation with Pt...Pt separations >6.3 A, and there are extensive close C-H...pi (d = 2.656-2.891 A), pi...pi (d = 3.322-3.399 A), and C-H...O=S (d = 2.265-2.643 A) contacts. [(Ph-C(wedge)N(wedge)C)Pt(PPh3)] (3) was prepared by reacting 2b with PPh3. Reactions of 2a-d with bis(diphenylphosphino)methane (dppm) gave [(R-C(wedge)N(wedge)C)2Pt2(mu-dppm)] (4a-d). Both head-to-head (syn) and head-to-tail (anti) conformations were found for 4a.6CHCl3.C5H12, whereas only one conformation was observed for 4b.2CHCl3 (syn), 4c.3CH2Cl2 (syn), and 4d.2CHCl3 (anti). In the crystal structures of 4a-d, there are close intramolecular Pt...Pt contacts of 3.272-3.441 A in the syn conformers, and long intramolecular Pt...Pt separations of 5.681-5.714 A in the anti conformers. There are weak C-H...X (d = 2.497-3.134 A) and X...X (X = Cl or Br; d = 2.973-3.655 A) interactions between molecules 4a-d and occluded CHCl3/CH2Cl2 molecules, and their solvent channels are of varying diameters (approximately 9-28 A). Complexes 2a-d, 3, and 4a-d are photoluminescent in the solid state, with emission maxima at 602-643 nm. Upon exposure to volatile organic compounds, 4a shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents (except CCl4). Powder X-ray diffraction analysis of desolvated 4a revealed a more condensed molecular packing of syn and anti complexes than crystal 4a.6CHCl3.C5H12.     

Molecular structure and conformational composition of 2-chloro-1-phenylethanone, ClH2C-C(=O)Ph, obtained using gas-phase electron-diffraction data and results from theoretical calculations

The structure and conformation of 2-chloro-1-phenylethanone, ClH(2)C-C(=O)Ph (phenacyl chloride), have been determined by gas-phase electron diffraction (GED), augmented by results from ab initio molecular orbital calculations, employing the second-order M?ller-Plesset (MP2) level of theory and the 6-311+G(d) basis set. The molecules may exist as a mixture of different conformers with the C-Cl bond either syn (torsion angle phi = 0 degrees ) or gauche to the carbonyl bond. At 179 degrees C, the majority of the molecules (90 +/- 11%) have the gauche conformation (phi = 112(3) degrees). Torsion is also possible about the C-Ph single bond. Both experimental and theoretical data indicated, however, that the phenyl ring is coplanar or nearly coplanar with the carbonyl group. The results for the principal distances (r(g)) and angles (angle(alpha)) for the gauche conformer from a combined GED/ab initio study (with estimated 2sigma uncertainties) are the following: r(C-C)(phenyl) = 1.394(2) (average value) A, r(C(phenyl)-C(carbonyl)) = 1.484(5) A, r(C(carbonyl)-C(alkyl)) = 1.513(5) A, r(C-Cl) = 1.790(5) A, r(C=O) = 1.218(6) A, r(C-H)(phenyl) = 1.087(9) (average value) A, r(C-H)(alkyl) = 1.090(9) A (average value), angle C(phenyl)-C=O = 119.5(9) degrees, angle C(phenyl)-C(carbonyl)-C(alkyl) = 119.2(10) degrees, angle C-C-Cl = 109.8(12) degrees, angle C(2)-C(1)-C(carbonyl) = 122.8(15) degrees, angle C-C(alkyl)-H = 111.2 degrees (ab initio value).     

1,N6‐Etheno‐2′‐deoxytubercidin hemihydrate

The title compound [systematic name: 7‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐7H‐imidazo[1,2‐c]pyrrolo[2,3‐d]pyrimidine hemihydrate], 2C₁₃H₁₄N₄O₃·H₂O or (I)·0.5H₂O, shows two similar conformations in the asymmetric unit. These two conformers are connected through one water molecule by hydrogen bonds. The N‐glycosylic bonds of both conformers show an almost identical anti conformation with χ = −107.7 (2)° for conformer (I‐1) and −107.0 (2)° for conformer (I‐2). The sugar moiety adopts an unusual N‐type (C3′‐endo) sugar pucker for 2′‐deoxyribonucleosides, with P = 36.8 (2)° and τ_m = 40.6 (1)° for conformer (I‐1), and P = 34.5 (2)° and τ_m = 41.4 (1)° for conformer (I‐2). Both conformers and the solvent molecule participate in the formation of a three‐dimensional pattern with a `chain'‐like arrangement of the conformers. The structure is stabilized by intermolecular O—H...O and O—H...N hydrogen bonds, together with weak C—H...O contacts.     

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.     

A 2′‐deoxycytidine long‐linker click adduct forming two conformers in the asymmetric unit

The title compound {systematic name: 4‐amino‐1‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐5‐[6‐(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)hex‐1‐ynyl]pyrimidin‐2(1H)‐one}, C₂₄H₂₈N₆O₄, shows two conformations in the crystalline state, viz. (I‐1) and (I‐2). The pyrimidine groups and side chains of the two conformers are almost superimposable, while the greatest differences between them are observed for the sugar groups. The N‐glycosylic bonds of both conformers adopt similar anti conformations, with χ = −168.02 (12)° for conformer (I‐1) and χ = −159.08 (12)° for conformer (I‐2). The sugar residue of (I‐1) shows an N‐type (C3′‐endo) conformation, with P = 33.1 (2)° and τ_m = 29.5 (1)°, while the conformation of the 2′‐deoxyribofuranosyl group of (I‐2) is S‐type (C3′‐exo), with P = 204.5 (2)° and τ_m = 33.8 (1)°. Both conformers participate in hydrogen‐bond formation and exhibit identical patterns resulting in three‐dimensional networks. Intermolecular hydrogen bonds are formed with neighbouring molecules of different and identical conformations (N—H...N, N—H... O, O—H...N and O—H...O).     

On the unusual 2J(C2-H(f)) coupling dependence on syn/anti CHO conformation in 5-X-furan-2-carboxaldehydes

A remarkable difference for (2)J(C(2)-H(f)) coupling constant in syn and anti conformers of 5-X-furan-2-carboxaldehydes (X = CH(3), Ph, NO(2), Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi-contact term transmission, a rather unusual dual-coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the J(H-H) in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the pi(C=C) electronic system. The homoallylic coupling pathway can be labeled as sigma*(C-H) <-- pi(C=C) --> sigma*(C-H). In the present case, this additional coupling pathway, using an analogous notation, can be labeled as sigma*(C(2)-C(C)) <-- LP(1)(O(1))...LP(2)(O(C)) --> sigma*(C(C)-H(f)) (sigma*(C(2)-C(C))) where O(1) and O(C) stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap.     

Ab initio study of the substituent effects on the relative stability of the E and Z conformers of phenyl esters. Stereoelectronic effects on the reactivity of the carbonyl group

Equilibria between the Z (tau1= 0 degrees) and E (tau1= 180 degrees) conformers of p-substituted phenyl acetates 4 and trifluoroacetates 5 (X = OMe, Me, H, Cl, CN, NO2) were studied by ab initio calculations at the HF/6-31G* and MP2/6-31G* levels of theory. The preference for the Z conformer, DeltaE(HF), was calculated to be 5.36 kcal mol(-1) and 7.50 kcal mol(-1) for phenyl acetate and phenyl trifluoroacetate (i.e., with X = H), respectively. The increasing electron-withdrawing ability of the phenyl substituent X increases the preference of the Z conformer. An excellent correlation with a negative slope was observed for both series between DeltaE of the E-Z equilibrium and the Hammett sigma constant. By using an appropriate isodesmic reaction, it was shown that electron-withdrawing substituents decrease the stability of both conformers, but the effect is higher with the E conformer. Electron-withdrawing phenyl substituents decrease the delocalization of the lone pair of the ether oxygen to the C=O antibonding orbital (nO--> pi*C=O) in both the E and Z forms and in both series studied; this effect is higher in the E conformer than in the Z conformer. The nO --> pi*C=O electron donation has a minimum value with tau1= 90 degrees and a maximum value with tau1= 0 degrees (the Z conformer), the value with tau1= 180 degrees (the E conformer) being between these two values, obviously due to steric hindrance. The effects of the phenyl substituents on the reactivity of the esters studied are discussed in terms of molecular orbital interactions. ED/EW substituents adjust the availability of the pi*C=O antibonding orbital to interact with the lone pair orbital of the attacking nucleophile and therefore affect the reactivity: EW substituents increase and ED substituents decrease it. Excellent correlations were observed between the rate coefficients of nucleophilic acyl substitutions and pi*C=O occupancies of the ester series 4 and 5.     

Structure and conformational properties of N-pentafluorosulfur(sulfuroxide difluoride imide) SF5N=S(O)F2: vibrational analysis, gas electron diffraction, and quantum chemical calculations

The molecular structure and conformational properties of N-pentafluorosulfur(sulfuroxide difluoride imide), SF5N=S(O)F2, have been studied by vibrational spectroscopy (IR (gas) and Raman (liquid)), by gas electron diffraction (GED), and by quantum chemical calculations (MP2 and B3LYP with (6-31G(d) and 6-311+G(2df) basis sets). According to GED, the prevailing conformer possesses a syn structure (N-SF5 bond synperiplanar with respect to the bisector of the SF2 group). Splitting of the symmetric N=S=O stretching vibration in gas and liquid spectra demonstrates the presence of a second conformer (11(5)%) with anticlinal orientation of the N-SF5 bond according to quantum chemical calculations. The geometric structure, conformational properties, and vibrational frequencies are well reproduced by quantum chemical calculations.     

Syntheses and structures of two low-dimensional beryllium phosphate compounds: [C5H14N2]2[Be3(HPO4)5].H2O and [C6H18N2]0.5[Be2(PO4)(HPO4)OH].0.5H2O

The first two low-dimensional beryllium phosphates, [C5H14N2]2[Be3(HPO4)5].H2O (BePO-CJ29) and [C6H18N2]0.5[Be2(PO4)(HPO4)OH].0.5 H2O (BePO-CJ30), have been successfully synthesized under mild hydrothermal/solvothermal conditions. BePO-CJ29 is built up from strict alternation of BeO4 and HPO4 tetrahedra forming a unique one-dimensional double chains with 12-ring apertures. There are pseudo-10-ring apertures enclosed by two double chains through H-bonds. BePO-CJ29 can also be viewed as a pseudo 2-D layered structure stabilized by strong H-bonds. The diprotonated 2-methylpiperazium cations are located at three positions (i.e., inside the 12-ring aperture, inside the pseudo-10-ring aperture, and in the interlayer of the inorganic pseudo-layers. BePO-CJ30 is constructed by the alternation of Be-centered tetrahedra (including BeO4 and HBeO4) and P-centered tetrahedra (including PO4 and HPO4) resulting in a two-dimensional layered structure parallel to the (0 1 1) direction. The complex layer is composed of coupled 4.8 net sheets. The diprotonated 1,6-hexandiamine cations and water molecules reside in the interlayer regions and interact with the inorganic layers through H-bonds. Crystal data are as follows: [C5H14N2]2[Be3(HPO4)5].H2O (BePO-CJ29), triclinic, P1 (No. 2), a = 8.1000(9) A, b = 8.4841(14) A, c = 19.665(2) A, alpha = 89.683(10) degrees, beta = 78.182(8) degrees, gamma = 87.932(9) degrees, V = 1321.9(3) A3, Z = 2, R1 = 0.0523 (I > 2sigma(I)), and wR2 = 0.1643 (all data); [C6H18N2]0.5[Be2(PO4)(HPO4)OH].0.5 H2O (BePO-CJ30), orthorhombic, Pccn (No. 56), a = 26.01(4) A, b = 8.431(12) A, c = 9.598(13) A, V = 2105(5) A3, Z = 8, R1 = 0.0833 (I > 2sigma(I)), and wR2 = 0.2278 (all data).     

Off-center oxygen-arene interactions in solution: a quantitative study

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.     

Anomeric and mesomeric effects in methoxycarbonylsulfenyl chloride, CH(3)OC(O)SCl: an experimental and theoretical study

The molecular structure and conformational properties of methoxycarbonylsulfenyl chloride, CH(3)OC(O)SCl, were determinated in the gas and solid phases by gas electron diffraction, low-temperature X-ray diffraction, and vibrational spectroscopy. Furthermore, quantum chemical calculations were performed. Experimental and theoretical methods result in structures with a planar C-O-C(O)-S-Cl skeleton. The electron diffraction intensities are reproduced best with a mixture of 72(8)% syn and 28(8)% anti conformers (S-Cl bond synperiplanar/antiperiplanar with respect to C=O bond) and the O-CH(3) bond synperiplanar with respect to the C=O bond. The syn form is the preferred form and becomes the exclusive form in the crystalline solid at low temperature. This experimental result is reproduced very well by Hartree-Fock approximation and by density functional theory at different levels of theory but not by the MP2/6-311G method, which overestimates the value of DeltaG between the syn and anti conformers. The results are discussed in terms of anomeric effects and a natural bond orbital (NBO) calculation. Photolysis of matrix-isolated CH(3)OC(O)SCl with broad-band UV-visible irradiation produces an interconversion of the conformers, and the concomitant decomposition leads to formation of OCS and CO molecules.     

Conformational studies of cyclopropane carboxaldehydehyde from temperature-dependent FT-IR spectra of xenon solutions

Variable temperature ( -60 to 100 degrees C) studies of the infrared spectra (3,500-400 cm(-1) of cyclopropane carboxaldehyde. c-C3H5CHO, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the syn and anti conformers. the enthalpy difference has been determined to be 95 +/- 8 cm(-1) (1.14 +/- 0.10 kJ/mol) with the (anti conformer (oxygen atom trans to the three-member ring) the more stable rotamer. From this deltaH value, along with assigned torsional transitions for both the anti and svi, conformers, the potential function governing the conformational interchange has been estimated. Using the new infrared data from the xenon solution, along with some additional Raman data, and ab initio predictions from MP2/6-31G(d) calculations some reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets upto 6-311 +/- +G(d,p) with full electron correlation by the perturbation method to second order from which structural parameters and conformational stabilities have been determined. From all of the ab initio calculations, the syn conformer is predicted to be the more stable which is at variance with the experimental results. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Structure and conformations of N-(fluoroformyl)imidosulfurous dichloride,FC(O)N=SCl2

The IR (gas) and Raman (liquid) spectra of FC(O)NSCl(2) demonstrate the presence of a conformational mixture in both phases. According to a gas electron diffraction study, the main conformer (94(8)%) possesses a syn-syn structure (C(O)F group synperiplanar with respect to the SCl(2) bisector and the C=O bond synperiplanar to the N=S bond). Quantum chemical calculations (HF, B3LYP and MP2 with 6-31G basis set, and MP2/6-311(2df)) predict a syn-anti structure for the second conformer. Analysis of the IR (gas) spectrum results in a contribution of 5(1)% of the minor form, corresponding to a Gibbs free energy difference DeltaG degrees = G degrees (syn-anti) - G degrees (syn-syn) = 1.75(15) kcal/mol. This value is reproduced very well by quantum chemical calculations, which include electron correlation effects (DeltaG degrees = 1.28-1.56 kcal/mol). The HF approximation overestimates this energy difference (DeltaG degrees = 3.24 kcal/mol).     

Molecular structure and conformational stability of allylisocyanate—post-Hartree–Fock and density functional theory studies

The molecular structure and conformational stability of allylisocyanate (CH₂CHCH₂NCO) molecule was studied using the ab initio and DFT methods. The geometries of possible conformers, C-gauche (δ=120°, θ=0°) (δ=C=C–C–N and θ=C–C–N=C) and C-cis N-trans (δ=0° and θ=180°) were optimized employing HF/6-31G^*, MP2/6-31G^* levels of theory of ab initio and BLYP, B3LYP, BPW91 and B3PW91 methods of DFT implementing the atomic basis set 6-311+G(d,p). The structural and physical parameters of the above conformers were discussed with the experimental and theoretical values of the related molecules, methylisocyanate and 3-fluoropropene. It has been found that the N=C=O bond angle is not linear as the experimental result for both the conformers and the theoretical bond angle is 173°. The rotational potential energy surfaces have been performed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The Fourier decomposition potentials were analysed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The HF/6-31G^* level of theory predicted that the C-gauche conformer is more stable than the C-cis N-trans conformer by 0.41 kJ/mol, but the MP2 and DFT methods predicted the C-cis N-trans conformer is found to be more stable than the C-gauche conformer. The calculated chemical hardness value at the HF/6-31G^* level of theory predicted the C-cis N-trans form is more stable than C-gauche form, whereas the chemical hardness value at the MP2/6-31G^* level of theory favours the slight preference towards the C-gauge conformer.