Stable Gauche Research Articles

Synergistic effect of intra- and intermolecular hydrogen bond in 2-haloethanols probed by infrared

Fluorine is often considered the only halogen to effectively engage in hydrogen bonds, while the other halogens, particularly iodine, are not electronegative enough to participate as hydrogen bond acceptors in electrostatic interactions. 2-Fluoroethanol and 2-iodoethanol have been studied herein to test this assumption, since a highly stable gauche conformation can experience the intramolecular hydrogen bond. However, the infrared O H stretching frequency indicates that the hydroxyl group in 2-fluoroethanol is not engaged in intramolecular hydrogen bond, while the corresponding vibration mode for 2-iodoethanol suggests that not only the O H is engaged in such interaction, but also that intramolecular hydrogen bond may drive the conformational equilibrium in this molecule. Theoretical calculations support the covalent nature of this interaction, and provide evidence that intermolecular hydrogen bond with a water molecule, and probably with the polar solvents tested experimentally, occurs with the hydroxyl rather than with the iodine substituent, as conventionally, in order to keep the intramolecular hydrogen bond effective.

Experimental/Computational Study on the Impact of Fluorine on the Structure and Noncovalent Interactions in the Monohydrated Cluster of ortho-Fluorinated 2-Phenylethylamine.

Fluorine-containing medicinal compounds frequently allow modulation of physical-chemical properties. Here, we address the effect of fluorine, near the ethylamino side chain, on conformational flexibility and noncovalent interactions (NCIs) of the selected jet-cooled monohydrated cluster of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) by mass-selected resonance-enhanced two-photon ionization and ionization-loss stimulated Raman spectroscopies. Our results show that Raman spectral signatures of the 2-FPEA-H2O cluster match the scaled harmonic vibrational Raman frequencies, resulting from density functional theory calculations of the most stable 2-FPEA gauche conformer hydrogen-bonded (HB) to water, confirming the three-dimensional cluster structure. This predicted electronic structure, together with NCI analysis, allows visualization and assessment of the attractive and repulsive interactions. The comparison of the NCIs and revealed red (O-H and N-H stretches) and blue shifts (C-H stretches and CH2 out-of-plane bends) of the cluster to other class members confirm O-H···N, N-H···π, C-H···O, and C-H···F HB formation and their contribution to structure stabilization, uncovering the potential of the approach.

Investigation of menopause-induced changes on hair by Raman spectroscopy and chemometrics

The ending of estrogen production in the ovaries after menopause results in a series of important physiologic changes, including hair texture and growth. In this study we demonstrate that Raman spectroscopy can be used successfully as a tool to probe menopause-induced changes on hair, in particular when coupled with suitable chemometrics approaches.The detailed analysis of the average Raman spectra (in particular of the Amide I and νS-S stretching spectral regions) of the hair samples of women pre- and post-menopause allowed to estimate that absence of estrogen in post-menopause women leads to an average reduction of ∼12% in the thickness of the hair cuticle, compared to that of pre-menopause women, and revealed the strong prevalence of disulphide bonds in the most stable gauche–gauche–gauche conformation in the hair cuticle. From the analysis of the νS-S stretching spectral region it could also be concluded that the amount of α-helix keratin is slightly higher for post-menopause than for pre-menopause women.A series of statistical models were developed in order to classify the hair samples. Outperforming the traditional PCA-LDA (principal component analysis – linear discriminant analysis) approach, in the present study a GA-LDA (genetic algorithm – linear discriminant analysis) strategy was used for variable reduction/selection and samples’ classification. This strategy allowed to develop of a statistical model (L16), which has exceptional prediction capability (total accuracy of 96.6%, with excellent sensitivity and selectivity) and can be used as an efficient instrument for the hair samples’ classification.In addition, a new chemometrics approach is here presented, which allows to overcome the intrinsic limitations of the GA algorithm and that can be used to develop statistical models that use GA as the variable reduction/selection method, but superseding its stochastic nature. Three suitable models for classification of the hair samples according to the menopause status of the women were developed using this novel approach (LV17, BLV20 and PLS7 models), which are based on the Fisher’s and Bayers’ LDA approaches and the PLS-DA method. The followed new chemometrics approach uses the results of a large set of GA-LDA runs over the full data matrix for the selection of the reduced data matrices. The criterion for the selection of the variables is their statistical significance in terms of number of occurrences as solutions of the whole set of GA-LDA runs.

Piloty’s acid and its hydrazide analogue: Insights from the density functional theory and vibrational spectroscopy on the conformational stability and chemical reactivity

N-hydroxybenzenesulfonamide (commonly known as Piloty’s acid) is considered a major source for nitroxyl (HNO) species which has potential biological and medicinal applications. In the present study, the conformational preferences and chemical reactivity of Piloty’s acid (PA) and its hydrazide analogue (benzenesulfonylhydrazide, BSH) were studied using spectroscopic and computational tools. Six stable conformations of each molecule were theoretically identified, and their structures were fully optimized at the DFT-B3LYP and MP2 levels. Both molecules in their most stable forms adopt the anti configuration with the NH bond of the secondary amine pointing away from the terminal hydroxyl and amine moieties in the acid and hydrazide molecules, respectively. Three stable gauche states facilitated by weak intramolecular interactions of the SO⋯HO and SO⋯HN types arise due to the internal rotation about the SN linkage. Reliable assignments of the vibrational modes and the calculated reaction coordinates support a two-step mechanistic pathway of the Piloty’s acid dissociation leading to the production of the nitroxyl (HNO) intermediate with moderate transition state barriers. Frontier molecular orbitals distributions, molecular electrostatic potential maps and condensed Fukui functions analysis of the molecules were employed to elucidate the agility of PA to dissociate to produce HNO and the absence of such a dissociation of BSH that would produce diazene (N2H2).

Effect of pH and Molecular Length on the Structure and Dynamics of Linear and Short-Chain Branched Poly(ethylene imine) in Dilute Solution: Scaling Laws from Detailed Molecular Dynamics Simulations.

Atomistic molecular dynamics (MD) simulations are carried out to examine the effect of molecular weight Mw (= 0.6, 0.86, 1.12, and 2.15 kDa) and pH (or equivalently, degree of ionization, α+ = 0, 50, and 100%) on the structure, state of hydration, and dynamics of linear and branched poly(ethylene imine) (PEI) chains in infinitely dilute salt-free aqueous solutions. It is found that the degree of ionization is the key factor determining the type of molecular conformation adopted by PEI, regardless of molecular architecture and chain length, resulting in a stable trans conformation for fully ionized solutions and in a stable gauche+/gauche- state for neutral or alternate ionized ones; in the latter case, a strong electrolyte behavior is verified for both linear and branched PEI. Linear PEI is observed to be significantly stiffer than branched PEI of the same molecular weight at 100% degree of ionization, but the effect subsides as the degree of ionization decreases. Also, linear PEI diffuses markedly slower than branched PEI of the same Mw. From the MD results, scaling exponents are deduced and reported for the conformation, solvent-accessible surface area, and dynamics of the two different PEI structures with Mw.

Influence of gauche effect on uncharged oxime reactivators for the reactivation of tabun-inhibited AChE: quantum chemical and steered molecular dynamics studies.

The neutral oxime reactivator RS194B with a seven-membered ring has shown better efficacy towards the tabun-inhibited AChE than that of RS69N with a six-membered ring and RS41A with a five-membered ring. The difference in the efficacy of these reactivators has remained unexplored. We report here the origin of the difference of efficacy of these reactivators based on the conformational analysis, quantum chemical calculations and steered molecular dynamics (SMD) simulations. The conformational analysis using B3LYP/6-31G(d) level of theory revealed that RS41A and RS194B are more stable in gauche conformation due to the gauche effect (-N-C-C-N- bonds) whereas RS69N prefers anti-conformation. The SMD simulations show that RS194B retains in more stable gauche conformation inside the active gorge of AChE during different time intervals that experiences more hydrogen bonding, hydrophobic interactions with the catalytic anionic site (CAS) residues and weaker interactions with the peripheral anionic site (PAS) residues compared to RS41A and RS69N. In an effort to design an even superior reactivator, RS194B-S has been chosen with a subtle change in the geometry of RS194B by replacing the carbonyl oxygen with the sulfur atom. The newly designed reactivator RS194B-S can also be a promising candidate to reactivate tabun-inhibited AChE.

Balanced design for the feasible super rocket fuels: A first-principle study on gauche CHN7 and CHN3

On the basis of the framework of cubic gauche nitrogen (cg-N), six one-eighth methanetriyl groups (>CH-) substitutes and fifteen one-fourth >CH- substitutes were optimized using the first-principle calculations based on density functional theory (DFT). Both one-eighth and one-fourth substitutes still keep the gauche structures with the simple formula CHN7 and CHN3, respectively. The most thermodynamic stable gauche CHN7 and CHN3 are P21 qtg-C2H2N14 I and P21 qtg-C4H4N12 III, respectively. No probability density of C-C single bonds and high probability densities of C-N-C structures were found in the two substitutes. Although gauche CHN7 and CHN3 lose energy density in contrast to cg-N, they win kinetic stability and combustion temperature (Tc). Thus, they are more feasible than cg-N, and more effective than the traditional rocket fuels.

Aluminum doping makes boron nitride nanotubes (BNNTs) an attractive adsorbent of hydrazine (N2H4)

Adsorption of toxic hydrazine (N2H4) at the surface of pristine and Al-doped single-wall boron nitride nanotubes (BNNTs and Al-BNNTs) has been investigated using density functional theory (DFT). Single hydrazine molecule was allowed to interact with Lewis acid center (i.e., B atom) of BNNTs, where most stable gauche conformer of the guest is used. Both zigzag (8,0) and armchair (4,4) variants of BNNTs were considered and to estimate the effect of tube diameter on adsorption energies wider zigzag (9,0) and (10,0), and armchair (5,5) and (6,6), tubes were included in the host list. Estimated adsorption energies of 2–8 kcal/mol do not support strong binding between pristine BNNTs with hydrazine. However, Al doping makes BNNTs an attractive adsorbent of N2H4 with adsorption energy in the range of 32–35 kcal/mol. Stability of such complexes is found less sensitive to the chirality, but nominally decreases with tube diameter. Thus, a large quantity of hydrazine may be adsorbed by Al-BNNT samples. The dative N: → B/Al bond between hydrazine and host tubes is the source of stability of complexes. Molecular electrostatic potentials (MEPs) and frontier molecular orbitals of host and guest were calculated to explain interaction character and strength.

The effects of conformation and zwitterionic tautomerism on the structural and vibrational spectral data of anserine

In this study, the stable conformers of neutral anserine were searched by molecular dynamics simulations and energy minimization calculations using the MM2 force field. Thermochemical calculations at B3LYP/6-31G(d) level of theory followed these preliminary calculations. The results confirmed that neutral anserine has quite a flexible structure and many stable gauche and trans conformers at room temperature. Nevertheless, two are considerably more favourable in energy than the others and expected to dominate the gas-phase and matrix IR spectra of the molecule. The corresponding structural and vibrational spectral data for these two conformers of neutral anserine, whose relative stabilities were also examined by high-accuracy energy calculations carried out using G3MP2B3 method, and for the most stable conformer of anserine in zwitterion form were calculated at B3LYP/6-311++G(d,p) level of theory. The calculated harmonic force constants were refined using the Scaled Quantum Mechanical Force Field (SQM-FF) method and then used to produce the refined wavenumbers, potential energy distributions (PEDs) and IR and Raman intensities. These refined data together with the scaled harmonic wavenumbers obtained using another method, Dual Scale factors (DS), enabled us to correctly analyse the observed IR and Raman spectra of anserine and revealed the effects of conformation and zwitterionic tautomerism on its structural and vibrational spectral data.

Weak intramolecular interaction effects on the torsional spectra of ethylene glycol, an astrophysical species.

An elaborate variational procedure of reduced dimensionality based on explicitly correlated coupled clusters calculations is applied to understand the far infrared spectrum of ethylene-glycol, an astrophysical species. This molecule can be classified in the double molecular symmetry group G8 and displays nine stable conformers, gauche and trans. In the gauche region, the effect of the potential energy surface anisotropy due to the formation of intramolecular hydrogen bonds is relevant. For the primary conformer, stabilized by a hydrogen bond, the ground vibrational state rotational constants are computed to be A0 = 15 369.57 MHz, B0 = 5579.87 MHz, and C0 = 4610.02 MHz corresponding to differences of 6.3 MHz, 7.2 MHz, and 3.5 MHz from the experimental parameters. Ethylene glycol displays very low torsional energy levels whose classification is not straightforward and requires a detailed analysis of the torsional wavefunctions. Tunneling splittings are significant and unpredictable due to the anisotropy of the potential energy surface PES. The ground vibrational state splits into 16 sublevels separated ∼142 cm(-1). The splitting of the "G1 sublevels" was calculated to be ∼0.26 cm(-1) in very good agreement with the experimental data (0.2 cm(-1) = 6.95 MHz). Transitions corresponding to the three internal rotation modes allow assignment of previously observed Q branches. Band patterns, calculated between 362.3 cm(-1) and 375.2 cm(-1), 504 cm(-1) and 517 cm(-1), and 223.3 cm(-1) and 224.1 cm(-1), that correspond to the tunnelling components of the v21 fundamental (v21 = OH-torsional mode), are assigned to the prominent experimental Q branches.

Infrared spectrum of the cold ortho-fluorinated protonated neurotransmitter 2-phenylethylamine: competition between NH+π and NH+F interactions.

Halogenation of pharmaceutical molecules is a common tool to modify their physiological properties. The geometric, vibrational, and electronic properties of the ortho-fluorinated protonated neurotransmitter 2-phenylethylamine (oF-H+PEA) are characterized by infrared photodissociation (IRPD) spectroscopy in the NH stretch range using the messenger technique and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level to elucidate the drastic effect of site-specific ortho-fluorination. The IRPD spectra of cold oF-H+PEA-Rg dimers (Rg = Ne, Ar) are assigned to the most stable gauche conformer (Gf1) of oF-H+PEA, which benefits from both NH+π and NH+F interactions. A minor contribution (∼5%) of the slightly less stable Gf2 gauche conformer (E0 = +1.1 kJ mol-1) is also identified. Comparison of oF-H+PEA with unsubstituted H+PEA reveals a much stronger NH+π interaction in H+PEA resulting in a large red shift of the bonded NH stretch frequency. This behavior is confirmed by natural bond orbital (NBO) analysis and noncovalent interaction (NCI) calculations. The Rg ligand prefers a binding site at which it can maximize the interaction with the aromatic π electron system and the ammonium group. Although the intermolecular interactions with the Rg atoms can compete with the noncovalent intramolecular bonds, they induce only minor spectral shifts in the NH stretch range.

Conformational analysis and electronic interactions of some 2-ethylsulfinyl-(4′-substituted)-phenylacetates

The analysis of the infrared carbonyl bands of some 2-ethylsulfinyl-(4′-substituted)-phenylacetates bearing the substituents NO2(1), Cl(2), Br(3), H(4), Me(5) and OMe(6), along with B3LYP/6-31G(d,p), SM5.42R solvation model calculations and natural bond orbital (NBO) analysis was carried out. Theoretical data indicated the existence in the gas phase of three stable gauche conformers, whose relative abundances in condensed phase change at different extent in the series 1–6, depending on the solvent permittivity. The comparison between the IR spectra in solvents of increasing permittivity (from CCl4 to CH3CN) and the calculated SM5.42R data in solution allows a precise assignment of the experimental band components to the distinct conformers. The sum of the main relevant NBO orbital interactions energies does not match the calculated B3LYP relative energies of the three stable conformers of 1–6 in the gas phase. On the contrary, the short contact analysis indicates that the electrostatic interactions largely control their relative stability and play a basic role to determine the calculated νCO frequencies order. Moreover, as these contacts dictate the specific geometry assumed by each conformer, the electrostatic interactions determine their different solvation properties too, thus accounting for the experimental stability observed in solution.

Spectroscopic identification of ethanol-water conformers by large-amplitude hydrogen bond librational modes

The far-infrared absorption spectra have been recorded for hydrogen-bonded complexes of water with ethanol embedded in cryogenic neon matrices at 2.8 K. The partial isotopic H/D-substitution of the ethanol subunit enabled by a dual inlet deposition procedure enables the observation and unambiguous assignment of the intermolecular high-frequency out-of-plane and the low-frequency in-plane donor OH librational modes for two different conformations of the mixed binary ethanol/water complex. The resolved donor OH librational bands confirm directly previous experimental evidence that ethanol acts as the O⋯HO hydrogen bond acceptor in the two most stable conformations. In the most stable conformation, the water subunit forces the ethanol molecule into its less stable gauche configuration upon dimerization owing to a cooperative secondary weak O⋯HC hydrogen bond interaction evidenced by a significantly blue-shift of the low-frequency in-plane donor OH librational band origin. The strong correlation between the low-frequency in-plane donor OH librational motion and the secondary intermolecular O⋯HC hydrogen bond is demonstrated by electronic structure calculations. The experimental findings are further supported by CCSD(T)-F12/aug-cc-pVQZ calculations of the conformational energy differences together with second-order vibrational perturbation theory calculations of the large-amplitude donor OH librational band origins.

Spectroscopic and theoretical studies of some 4′-substituted-phenyl 2-(ethanesulfonyl)acetates. Structure of 4′-nitrophenyl 2-(ethanesulfonyl)acetate

Abstract An analysis of carbonyl bands in the infrared spectra of some 2-ethylsulfonyl 4′-substituted phenylacetates bearing substituents NO2 (1), H (2) and OMe (3), supported by B3LYP/6-31G(d,p) and SM5.42R at PM3 level calculations, along with natural bond orbital analysis (NBO) and X-ray diffraction (for 1) was performed. Theoretical data indicated the existence of two stable gauche conformations (g 1 and g 2). The g 1 conformer is the most stable, least polar and has the lowest νCO frequency. The more intense, lower frequency carbonyl doublet component found in CCl4 solution is assigned to the g 1 conformer. As the solvent dielectric constant increases (going from CCl4 to MeCN) the higher frequency νCO doublet increases in intensity. This behaviour is reproduced by the solvation free energy calculations, supporting the conformer assignments. NBO calculations indicate that the most important orbital interaction is LPO9 → π*C7=O8 for both conformers, which corresponds to [C=O ↔ C+–O-] conjugation. This stabilises the g 1 conformer to a greater extent and is responsible for the lower νCO frequency. The sum of the selected NBO delocalisation energies for 1–3 indicates that the g 1 conformer is more stable. It is concluded that the calculated greater stability of the g 1 conformer is due to a balance of attractive electrostatic and orbital interactions along with relevant hydrogen bonds. The X-ray crystal structure analysis of 1 shows the presence of two crystallographic independent but almost superimposable molecules each which adopt a cis geometry. The molecules are consolidated into the three-dimensional crystal packing by C–H…O interactions as well as by nitro- N–O…π(phenyl) contacts.

Conformational stability, r0 structural parameters, vibrational assignments and ab initio calculations of ethyldichlorophosphine

Variable temperature (−60 to −100°C) studies of ethyldichlorophosphine, CH3CH2PCl2, of the infrared spectra (4000–400cm−1) dissolved in liquid xenon have been carried out. From these data, the two conformers have been identified and the enthalpy difference has been determined between the more stable trans conformer and the less stable gauche form to be 88±9cm−1 (1.04±0.11kJ/mol). The percentage of abundance of the gauche conformer is estimated to be 57% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing many 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 both conformers which have been predicted by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, wavenumbers of the fundamentals, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared to the corresponding properties of some related molecules.

Microwave, infrared and Raman spectra, adjusted r0 structural parameters, conformational stability, and vibrational assignment of cyclopropylfluorosilane

FT-microwave, infrared spectra of gas and Raman spectra of liquid for cyclopropylfluorosilane, c-C3H5SiH2F have been recorded. 51 transitions for the 28Si, 29Si, and 30Si isotopomers have been assigned for the gauche conformer. Enthalpy differences in xenon solution by variable temperature infrared spectra between the more stable gauche and lesser stable cis form gave 109±9cm−1. From the microwave rotational constants for the three isotopomers (28Si, 29Si, 30Si) combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters were obtained for the gauche conformer. The heavy atom distances (Å): Si–C2=1.836(3); C2–C4=1.525(3); C2–C5=1.519(3); C4–C5=1.494(3); Si–F=1.594(3) and angles (°): ∠CSiF=111.2(5); ∠SiC2C4=117.5(5); ∠SiC2C5=119.2(5). To support the vibrational assignments, MP2(full)/6-31G(d) calculations were carried out. Results are discussed and compared to the corresponding properties of some similar molecules.

Molecular conformational structures of 2-fluorobenzoyl chloride, 2-chlorobenzoyl chloride, and 2-bromobenzoyl chloride by gas electron diffraction and normal coordinate analysis aided by quantum chemical calculations

The gas phase molecular structures and conformational compositions of 2-fluorobenzoyl chloride, 2-chlorobenzoyl chloride, and 2-bromobenzoyl chloride have been investigated using gas electron diffraction data obtained from experiments performed in the laboratories of the University of Oslo and Oregon State University. The refinements on the experimental data have been aided by normal coordinate calculations as well as extensive ab initio molecular orbital and density functional theory calculations up to the levels of MP4(SDQ) and B3LYP with larger basis sets up to the level of 6-311 + G(2d,p) for the computed molecular geometries, electronic energies, vibrational zero-point energies and entropy corrections, gas mixture conformational compositions, and MP2(fc) quantum mechanical force fields. The three title molecules each exist in the gas phase as two stable non-planar conformers anti and gauche with respect to the halogen atom positions with anti the lower energy conformer in each case. Among the three title molecules there have been found considerable experimental and theoretical support for several trends in molecular or conformational behavior with increasing ortho halogen atomic size: An increasing although disputable trend in the C=O bond distance values; an increasing trend in the average phenyl ring C–C bond distance values; an increasing trend in the contribution of the gauche conformer to the gaseous mixture lowering the standard free energy difference values (ΔG o) correspondingly; and an increasing deviation from full planarity (C s symmetry) in both the anti and the gauche conformers of the title molecules with increasing ortho halogen atomic size. Only in the anti conformer of 2-fluorobenzoyl chloride does the experimental data refinements suggest close to full planarity for these 2-halobenzoyl chloride molecules.

Structural Changes of Carotenoid Astaxanthin in a Single Algal Cell Monitored in Situ by Raman Spectroscopy

The changes of structure of astaxanthin (AXT), a superpotent antioxidant, upon thermal stress were investigated in unicellular microalgae Haematococcus pluvialis by measuring Raman spectra in situ and analyzing obtained results with DFT calculations. Although no visual changes are observed in the Haematococcus cells upon heating, discernible changes in Raman spectra occur from -100 °C systematically up to 150 °C. The exponential increase of the Raman shift of the ν C═C band at ca. 1520 cm(-1) along with the change of the intensity ratio of bands at 1190 and 1160 cm(-1) is observed, that correlates with the changes predicted by calculations for astaxanthin conformers ordered by decreasing energy. It is assumed that AXT molecules, initially in the form of H-aggregates with the trans conformations of the end-rings, interconvert toward more stable gauche forms upon thermal stress of the algae. The applied approach enables one to follow structural changes of the carotenoid upon temperature stress both in a single algal cell and in a multicellular sample in situ. Obtained information might be of use to improve the industrial process of extraction of AXT in its most bioavailable form.

Spectroscopic and theoretical studies of some N, N-diethyl-2-[(4′-substituted)phenylsulfonyl]acetamides

The analysis of the IR carbonyl band of the N, N-diethyl-2-[(4′-substituted)phenylsulfonyl]acetamides Et 2NC(O)CH 2S(O) 2 C 6H 4 Y (Y = OMe 1, Me 2, H 3, Cl 4, Br 5, NO 2 6) supported by B3LYP/6-31G(d,p) calculations for 3, indicated the existence of three pairs ( anti and syn) of cis ( c) and gauche ( g 1 and g 2) conformers in the gas phase, being the gauche conformers significantly more stable than the cis ones. The anti geometry is more stable than the syn one, for each pair of cis and gauche conformers. The summing up of the orbital (NBO analysis) and electrostatic interactions justifies quite well the populations and the ν CO frequencies of the anti and syn pairs of c, g 1 and g 2 conformers. The IR higher carbonyl frequency component whose population is ca. 10%, in CCl 4, may be ascribed to the least stable and most polar cis conformer pair (in the gas phase) and the lower frequency component whose population is ca. 90%, to the summing up of the populations of the two most stable and least polar gauche conformer pairs ( g 1 and g 2) (in the gas phase). The reversal of the cis( c)/ gauche ( g 1 + g 2) population ratio observed in chloroform ca. 60% ( cis)/40% ( gauche) and the occurrence of the most polar cis( c) conformer only, in acetonitrile, strongly suggests the coalescence of the two gauche components in a unique carbonyl band in solution. A further support to this rationalization is given by the single point PCM solvation model performed by HF/6-31G(d,p) method, which showed a progressive increase of the c/( g 1 + g 2) ratio going from gas to CCl 4, to CHCl 3 and to CH 3CN. X-ray single crystal analysis of 4 indicates that this compound assumes, in the solid state, the syn-clinal ( gauche) conformation with respect to the [O C CH 2 S] moiety, and the most stable anti geometry relative to the [C(O)N(CH 2CH 3) 2] fragment. In order to obtain larger energy gain from the crystal packing the molecules of 4 are linked in centrosymmetric dimers through two C H⋯O interactions (C H [O–Ph]⋯O [SO2]) forming a step ladder.

Structure and Torsional Properties of Oxalyl Chloride Fluoride in the Gas Phase: An Electron-Diffraction Investigation

The structure and torsional properties of oxalyl chloride fluoride in the gas phase have been measured by electron diffraction at temperatures of 22, 81, 158, and 310 °C. The molecule may be regarded as a hybrid of oxalyl chloride and oxalyl fluoride. Since the former exists as a more stable periplanar anti form (ϕ = 180°) in equilibrium with a less stable gauche form (ϕ ≃ 60°) and the latter as an equilibrium between two periplanar forms, anti and syn, the second form of oxalyl chloride fluoride is an interesting question. It was found to be gauche. The system was modeled as two rotational conformers related by a potential of the form 2V = V(1)(1 + cos ϕ) - V(2)(1 - cos 2ϕ) + V(3)(1 + cos 3ϕ). The anti/gauche bond distances and bond angles (r(g)/Angstroms, ∠(α)/degrees) with estimated 2σ uncertainties at 22 °C are <r(C═O)> = 1.183(2)/1.182(2), Δr(C═O) = 0.003(6)/0.002(6) (assumed from theory), r(C-F) = 1.329(3)/1.335(3), r(C-Cl) = 1.738(2)/1.753(2), ∠(C-C-Cl) = 112.0(3)/111.9(3), ∠(C-C═O3) = 123.0(4)/123.2(4), ∠(O═C-Cl) = 125.0(2)/1.249(2), ∠(O═C-F) = 123.0(3)/125.1(3), and ∠(Cl-C-C-F) = 180.0/59.8. The variation of composition with temperature afforded a determination of the standard enthalpy and entropy of the reaction anti → gauche. The results are ΔH° = 2.5(12) kcal/mol and ΔS° = -6.5(33) cal/(mol·K). The structures and equilibria are discussed.