A hydrogen bonded complex C2H2…HBr isolated and characterized in the gas phase using pulsed-jet,Fourier transform microwave spectroscopy

A complex formed from ethyne and hydrogen bromide has been isolated and characterized by using a fast-mixing nozzle in conjunction with a pulsed-jet, Fourier transform microwave spectrometer. Any possible chemical reaction between the two components when mixed in the usual way was thereby precluded. The rotational constants A, B and C, the quartic centrifugal distortion constants δ_J, δ_Jk and δ_J and the Br nuclear hyperfine coupling constants X_aa and X_bb—X_cc (nuclear quadrupole) and M_bb (spin-rotation) were determined for each of the five isotopomers C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹ Br, C₂H₂…D⁷⁹Br, C₂H₂…D⁸¹Br and C₂D₂…H⁷⁹Br. Interpretations of the spectroscopic constants show that the complex is planar and T shaped in the equilibrium conformation, with HCCH internuclear axis forming the cross of the T and the HBr internuclear axis lying along the C₂ axis of C₂H₂. The H of HBr is closer to the centre (?) of the π bond of ethyne, and therefore HBr is involved in a hydrogen bond to the π system, in which the distance of H from ? is r (?…H) = 2.469(1) A. The intermolecular stretching force constant is estimated as kσ = 5.38(2) Nm^?1 for the species involving a hydrogen bond (C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹Br and C₂D₂…H⁷⁹Br) while this quantity increases to kσ = 5.68(2)Nm^?1 for those complexes bound through a deuterium bond (C₂H₂…D⁷⁹Br and C₂H₂…D⁷⁹Br). The opportunity is taken to consider similarities in the properties of complexes within the two series B…HBr and B…HC1 for a range of Lewis bases B, including B = C₂H₂. Some family relationships are identified in the two series.     

Nitrosation of dialkylhydroxylamines,and computational and NMR investigations of the interconversion of conformational isomers of N‐nitroso‐dimethylhydroxylamine

The effect of acidity upon the rate of nitrosation of N‐benzyl,O‐methylhydroxylamine ( 3 ) in 1:1 (v/v) H₂O/MeOH at 25 °C has been investigated. The pseudo‐first‐order rate constant (k_obs) for loss of HNO₂ as the limiting reagent decreases as [H₃O⁺] increases. This is compatible with two parallel reaction channels (Scheme 2 ). One involves the direct reaction of the free hydroxylamine with HNO₂ (k₁ = 1.4 × 10² dm³ mol^?1 s^?1, 25 °C) and the other involves the reaction of the free hydroxylamine with NO⁺ (k₂ = 5.9 × 10⁹ dm³ mol^?1 s^?1). In contrast, there is only a very slight increase in k_obs with increasing [H₃O⁺] for nitrosation of N,O‐dimethylhydroxylamine ( 4 ) in dilute aqueous solution at 25 °C to give N‐nitroso‐dimethylhydroxylamine, 5 . This also fits a two‐channel mechanism (Scheme 3 ). Again, one involves the nitrosation of the free base by NO⁺ (k₂ = 8 × 10⁹ dm³ mol^?1 s^?1, 25 °C) but the other channel now involves catalysis by chloride (k₃ = 1.3 × 10⁸ dm³ mol^?1 s^?1). Arising from these results, we propose an estimate of pK_a ～ ?5 for protonated nitrous acid, (O = N? OH), which is appreciably different from the literature value of +1.7. The interconversion of cis and trans conformational isomers of 5 has been investigated by temperature‐dependent NMR spectroscopy in CDCl₃, methanol‐d₄, toluene‐d₈ and dimethyl sulfoxide‐d₆. Enthalpies and entropies of reaction and of activation have been determined and compared with computational values obtained at the B3LYP/6‐31G* level of theory. The cis form is slightly more stable at normal temperatures and no solvent effects upon the thermodynamics or kinetics of the conformational equilibrium were predicted computationally or detected experimentally. In addition, key geometric parameters and dipole moments have been calculated for the cis and trans forms, and for the lowest energy transition structure for their interconversion, in the gas phase and in chloroform. These results indicate electronic delocalisation in the ground states of 5 which is lost in the transition structure for their interconversion. Copyright © 2010 John Wiley & Sons, Ltd.     

Anharmonic force fields of cis- and trans-S1 C2H2

We calculate second-order vibrational perturbation theory (VPT2) anharmonic force fields for the cis and trans conformers of S₁ C₂H₂, and compare the results to experiment. The vibrational assignments of recently observed levels belonging to the cis well are of particular interest. A refined estimate of the cis origin position (44,870?±?10?cm^?1) is proposed, and preliminary low-energy fits to the global J?=?K?=?0 trans level structure are also described. The performance of perturbation theory in this isomerizing system is examined, and both surprising successes and failures are encountered. We examine these and their causes, and offer practical suggestions for avoiding the pitfalls of applying perturbation theory to systems with large amplitude motions.     

Infrared diode laser spectroscopy of the Kr–N2O van der Waals complex: the v 1 symmetric stretch region of N2O

The rovibrational spectra of four isotopomers of the Kr–N₂O van der Waals complex, namely ⁸²Kr–N₂O, ⁸³Kr–N₂O, ⁸⁴Kr–N₂O and ⁸⁶Kr–N₂O, were measured in the v ₁ vibrational band region of the N₂O monomer (～1285?cm^?1) using a tunable diode laser spectrometer to probe a pulsed supersonic slit jet. Rotational constants for both ground and excited vibrational states of these four isotopomers were accurately determined. The band-origin of Kr–N₂O was observed to shift by +0.1065?cm^?1 from that of the monomer. The band-origin shifts of Rg–N₂O (Rg?=?Ne, Ar, Kr) in the v ₁ vibrational band region could also be well explained by the model based on a Buckingham intermolecular potential [W.A. Herrebout, H.-B. Qian, H. Yamaguchi and B.J. Howard, J. Mol. Spectrosc. 189, 235 (1998)]. But the band-origin shift of He–N₂O was found to deviate significantly from this model. The possible reason is discussed and the band-origin shift of Xe–N₂O predicted.     

Substituent effects on the conformational stability of allyl fluorides

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

Structures and Vibrational Spectra of the Hydrogen-Bonded Complexes Between Nitrous Acid and Acetone: Ab Initio and DFT Studies

ABSTRACT

The structures, stability, and vibrational spectra of the binary complexes formed between acetone and nitrous (trans and cis) acid have been investigated using ab initio calculations at the SCF and MP2 levels and B3LYP calculations with 6-311++G(d,p) basis set. Full geometry optimization was made for the complexes studied. It was established that the complex (CH₃)₂CO···HONO-trans is more stable than the complex (CH₃)₂CO···HONO-cis by 0.5–0.8 kcal·mol^?1. The accuracy of the calculations has been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP/6-311++G(d,p) calculations are very near to the results, obtained with MP2/6-311++G(d,p). The calculated frequency shift Δν(O[sbnd]H) for the complex (CH₃)₂CO···HONO-trans (1A) is larger than for the complex (CH₃)₂CO···HONO-cis (1B). In the same time the intensity of this vibration increases dramatically upon hydrogen bonding. The calculated increase for the complex 1A is up to 15 times and for the complex 1B is up to 30 times. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) of (CH₃)₂CO upon the complexation are more insignificant than the changes in the vibrational characteristics of HONO-trans and HONO-cis.     

Ab initio determination of the torsional spectra of acetic acid

The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-p VTZ ab initio calculations. The molecule shows two planar trans and cis conformers whose energy difference is 1882.7 cm^?1. Both minimum energy geometries are separated by a barrier of 4432.1 cm^?1. The most stable trans-conformer shows a quite low methyl torsion barrier of 169.8 cm^?1. The roto-torsional energy levels have been calculated up to J = 10. The two torsional fundamental frequencies of the trans-conformer, the methyl and the OH torsion are 82.857 (A²) and 77.050cm^?1 (E) and 568.532 (A²) and 568.418cm^?1 (E). The V₃ barrier causes a splitting of 0.315cm^?1 in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be D_J = 90.4kHz, D_JK = ?301.5kHz and D_K = 165.4kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.     

The integrated number of vibrational states in acetylene (12C2H2, 13C2H2, 12C2D2)

A calculated exhaustive set of vibrational state energies in ¹²C₂H₂, ¹³C₂H₂ and ¹²C₂D₂ has been used to analyse the evolution of the integrated number of states with increasing vibrational energy N(E) up to 15000 cm^?1, 12000cm^?1 and 10000 cm^?1 in each isotopomer, respectively. The regular contribution to N(E) was modelled analytically and numerical parameters were fitted. The other expected contribution to N(E), which is of oscillatory nature, was quantified and is discussed using energyand time-dependent theories. Related periods of oscillation and temporal recurrences are interpreted consistently in terms of the constant of the motion N_r = 5v₂ + 3v₂ + 5v₃ + v₄ + v₅ and of an average vibrational quantum. More pragmatically, the vibrational dynamics appear to be dominated by the bending vibrations, i.e., by the slowest oscillators.     

Structures,vibrational spectra,and relative energetics of FC(O)ONO and FC(O)NO2 isomers at DFT and ab initio levels

The structural and vibrational parameters of FC(O)ONO and FC(O)NO₂ isomers were examined theoretically using the B3LYP/6-311+G(3df) and CCSD(T)/6-311G(d) methods. Four conformers of FC(O)ONO isomer and one FC(O)NO₂ isomer are found here. Among them, the trans–cis and cis–cis FC(O)ONO configuration are new conformers. The energetics were refined with G3//B3LYP and CBS-QB3 calculations. The trans–trans conformer of the FC(O)ONO isomer is found to be the lowest energy structure, with an estimated heat of formation of ?104.9 kcal mol^?1 at 0 K as determined from CBS-QB3 theory. The next lowest structure is the cis–trans FC(O)ONO lying 1.7 kcal mol^?1 above the trans–trans structural form. The highest energy structure is the FC(O)NO₂ isomer with a predicted heat of formation of ?84.8 kcal mol^?1. A comparison of the relative stability of the FCNO₃ isomers with the isomers of ClCNO₃ shows that the Cl analogues follow the same pattern of stability, as do the F isomers. However, the chlorine isomers are unstable relative to their fluorine analogues.     

Acetylene, 12C2H2: new CRDS data and global vibration-rotation analysis up to 8600 cm−1

The absorption spectrum of ¹²C₂H₂ has been recorded using cavity ringdown spectroscopy and analyzed in the ranges 6000 ? 6356 cm^?1 and 6667?7015 cm^?1. Fourteen new bands have been identified and additional J-lines were assigned in 10 already known bands. These new data, together with the published vibration–rotation absorption lines of ¹²C₂H₂ accessing vibrational states up to 8600 cm^?1 have been gathered in a database. The resulting set includes 12137 transitions involving 186 different k = l ₄ + l ₅ sub-states, with l _i the angular momentum quantum number associated to the i degenerate bending vibration. These lines have been fitted simultaneously to spectroscopic parameters through J-dependent Hamiltonian matrices exploiting the vibrational polyad or cluster block-diagonalization, in terms of the pseudo-quantum numbers N_r = 5v ₁ + 3v ₂ + 5v ₃ +v ₄ +v ₅ and N_s = v ₁ + v ₂ + v ₃, also accounting for k and e/f parities and u/g symmetry. Modes 1 and 2 correspond to the symmetric CH and CC stretchings, mode 3 to the antisymmetric CH stretch, and modes 4 and 5 to the trans- and cis-degenerate bendings, respectively. The fit was successfully achieved, with a dimensionless standard deviation of 0.92, leading to the determination of 266 effective vibration-rotation parameters.     

Photochemical preparation of nitrosomethanol: Vibrational frequencies,force field,and normal coordinate analysis of the cis and trans isomers

The previously unknown molecule C-nitrosomethanol [H₂C(OH)(NO)] and seven of its isotopic modification [¹³C, ¹⁸OH, ¹⁵N, D₃, D₃-¹⁵N, H₂C(OD)(NO), and HDC(OH)(NO)] have been photolytically prepared from the corresponding methylnitrite isotopes in a low-temperature argon matrix. By irradiating methylnitrite into its $\text{S}{}_1\text{(nπ}{}^1\text{← S}{}_0$ transition at 365 nm, the molecule is transformed into a hydrogen-bonded 1:1 complex between formaldehyde and nitroxyl. Photolysis of this complex produces either the trans (λ_exc = 345 nm) or the cis (λ_exc ≥ 645 nm) isomer of nitrosomethanol. Selective photoisomerization processes permit interconversion of the conformers (cis → trans with λ_exc = 510 nm and trans → cis with λ_exc ≥ 645 nm). The ir spectra (40–170 cm^?1) of cis- and trans-nitrosomethanol were measured and analyzed in conjunction with a normal coordinate analysis based on a constrained valence force field (transferable valence force field approach). The frequencies of cis- and trans-nitrosomethanol were assigned and their force fields determined.     

Enthalpies of formation and isomerization of cis‐ and trans‐decalin,and their oxa‐analogs by G3(MP2)//B3LYP calculations

G3(MP2)//B3LYP calculations have been carried out on trans‐ and cis‐decalin, and their mono‐, di‐, tri‐, and tetraoxa‐analogs. The main purpose of the work was to obtain enthalpies of formation for these compounds, and to study the relative stabilities of the cis–trans and positional isomers of the various (poly)oxadecalins. Comparison of the computational enthalpies of formation with the respective experimental ones, known only for the decalins and 1,3,5,7‐tetraoxadecalins, shows that in both cases the computational values are more negative than the experimental ones, the deviations being ?5 to ?7 kJ mol^?1 for the decalins and ?12 to ?17 kJ mol^?1 for the 1,3,5,7‐tetraoxadecalins. The respective computational enthalpies of cis–trans isomerization, however, are in excellent to satisfactory agreement with the experimental data. The cis–trans enthalpy differences vary from +11.0 kJ mol^?1 for decalin to ?15.4 kJ mol^?1 for 1,4,5,8‐tetraoxadecalin. Low relative enthalpy values were also calculated for the cis isomers of 1,8‐dioxadecalin (?3.7 kJ mol^?1), 1,3,6‐trioxadecalin (?4.6 kJ mol^?1), 1,3,8‐trioxadecalin (?9.7 kJ mol^?1), 1,4,5‐ trioxadecalin (?5.6 kJ mol^?1), 1,3,5,8‐tetraoxadecalin (?7.3 kJ mol^?1), and 1,3,6,8‐tetraoxadecalin (?14.5 kJ mol^?1). Copyright © 2009 John Wiley & Sons, Ltd.     

Diffusion of small penetrant molecules in polybutadienes

The diffusion coefficient D in the dilute limit for three different penetrants – oxygen, water, and methanol – in three different conformations of polybutadiene (all cis-1,4, all trans-1,4, and a random copolymer containing 50% trans-1,4, 40% cis-1,4, and 10% vinyl-1,2 repeat units) has been computed using molecular dynamics simulations for temperatures in the range T?=?300–400?K. Simulation runs of 25 and 50?ns were made for each of the 45 combinations of penetrant, conformation, and temperature studied. Over this temperature range the density of the all-cis-1,4 conformation is higher than that of the all-trans-1,4 and random copolymer conformations, which are approximately equal. For all three conformations, D for oxygen and water are comparable and larger than that of methanol. However for a given penetrant, strong differences were observed in the rate of increase of D for the three conformations. We find that the activation barriers for the three penetrants are generally between 20 and 30?kJ?mol^?1, in agreement with experimental results. The magnitude of the activation energy is directly proportional to the size, rather than the mass, of the penetrant molecule.     

A computational characterization of N2@C60

Recent observations of N₂@C₆₀ are supported computationally. The geometry is optimized at the B3LYP/3-21G and PW91/3-21G levels. The lowest-energy structure has the N₂ unit oriented towards a pair of parallel pentagons so that the complex exhibits D_5d symmetry. Single-point energy calculations are further carried out at the B3LYP/6-31G*, PW91/6-31G* and MP2?=?FC/6-31G* levels and corrected for the basis set superposition error (BSSE). The MP2?=?FC/6-31G* treatment with the BSSE correction gives a stabilization energy of -9.3?kcal?mol^?1, whereas DFT approaches mostly fail to produce a stabilization. The entropy term for the encapsulation is also evaluated and leads to a standard Gibbs energy change upon encapsulation at room temperature of -3.3?kcal?mol^?1. The computed structural and vibrational characteristics are also reported.     

Die Eigenschwingungen des Metall-Hydratkomplexes in BeSO4·4H2O und BeSO4·4D2O

Infrared reflection spectra of single crystals of BeSO₄·4H₂O and BeSO₄·4D₂O have been obtained in polarized light at 300°K and at 14°K in the region between 4000 cm^?1 and 300 cm^?1. By a Kronig-Kramers analysis, the frequencies of the infrared active transitions have been calculated. These transitions are attributed to internal vibrations of the water molecules and sulfate ions and, in the region between 1000 cm^?1 and 300 cm^?1, especially to internal and external vibrations of the tetrahedral Be⁺⁺·4aqu-complexes. The vibrational modes of these complexes consist of a superposition of translational and librational modes of the water molecules and translational modes of the central Be⁺⁺-ion. The vibrational frequencies and normal modes of this complex have been calculated in a central-force model, and force-constants have been determined by fitting the calculated frequencies to the observed spectra. The calculations have shown that the modes, which comprise mainly translational motions of the water molecules, are strongly coupled with librational motions of the water molecules. On the other hand, there exist pure librational modes with practically no admixture of translational motions. The optimum sets of force constants for the BeSO₄·4H₂O crystal and the BeSO₄·4D₂O crystal differ in a manner which can be understood under the assumption that the dimensions of the Be(D₂O)₄ complex are about 0.1 Å larger than those of the Be(H₂O)₄ complex. Some arguments supporting this conclusion will be discussed.     

Irreversible cis-trans-Photoisomerization of Porphyrin Dimers with an Ethylene Bridge

On the basis of a comparison of the stationary absorption, fluorescence, and fluorescence excitation spectra of solutions of synthetic dimers of octaethylporphyrin (H₂OEP) and its zinc complex (ZnOEP) of composition ZnOEP–ZnOEP and ZnOEP–H₂OEP linked by an ethylene bridge in cis- and trans-geometry, we conclude that the cis-isomers of the above dimers, when illuminated, irreversibly change to their corresponding trans-isomers.     

Hydration-induced conformational changes in protonated 2,4-pentanedione in the gas phase

Starting with H⁺[CH₃C(O)CH₂C(O)CH₃] (denoted H⁺PD), the protonated diketone-water clusters H⁺PD(H₂O) _n (n = 1–3) have been characterized by density functional theory calculations in combination with vibrational predissociation spectroscopy to explore the conformational changes of a protonated bifunctional ion solvated by water in the gas phase. Theoretical calculations for H⁺PD revealed that the ion contains an intramolecular hydrogen bond (IHB), with two oxygen atoms bridged by the extra proton in an O—H⁺ … O form. Attachment of one water molecule to it readily ruptures this IHB, replacing the H⁺ by the H₃O⁺ moiety. Further replacement of the IHB by two water molecules occurs at n = 2 and the ?C(O)CH₂C(O)- chain is fully opened (or unfolded) after transfer of the extra proton to the water trimer at n = 3. To verify the computational findings, infrared spectroscopic measurements were performed using a vibrational predissociation ion trap spectrometer to identify cluster isomers from the signatures of hydrogen bonded and non-hydrogen bonded OH stretching spectra of H⁺PD(H₂O)_2,3 produced in a corona discharge supersonic expansion. Besides open form isomers, evidence for the formation of water-bridged structures has been found for H⁺PD(H₂O)₃ at an estimated temperature of 200 K. A detailed illustration of the unfolding steps as well as the energy profiles for the evolution of a two-water bridge isomer from the protonated H⁺PD monomer are analysed pictorially (including both stable intermediates and transition states) in the present investigation.     

The concentration dependence of the proton chemical shift and the deuterium quadrupole coupling parameter for binary solutions of ethanol

Concentration dependent experimental measurements of the ethanol hydroxyl proton chemical shift σ_H for binary solutions were carried out. The solvents used were carbon tetrachloride (CCl₄), benzene, chloroform, acetonitrile, acetone and dimethylsulphoxide (DMSO). The chemical shift values range from 0.69 ppm (relative to TMS) for dilute ethanol (extrapolated to infinite dilution) in CCl₄ to 5.34 ppm for neat liquid ethanol. Ab initio calculations of the ethanol-solvent hydrogen bond energies show a correlation with the values for the chemical shift. The hydrogen bond energies for ethanol-solvent dimers range from 0.63 kcal mol^?1 for ethanol-CCl₄ to 9.34 kcal mol^?1 for ethanol-DMSO. Theoretical calculations show a linear correlation between the deuterium quadrupole coupling parameter XD ^ar d the isotropic proton chemical shift σ_H: X_D(kHz) = 291.48 ? 14.96 σ_H, where σ_H is the proton chemical shift in ppm relative to TMS (R ² = 0.99). Using the concentration dependent chemical shift data and this equation, X_D ia observed to range from 280 kHz for very dilute concentrations in CCl₄, where the primary species is ethanol monomer, to 210 kHz for the neat liquid that is comprised primarily of cyclic pentamers.     

Towards an understanding of the helium–acetylene van der Waals complex

The weakly bound complex He–C₂D₂ is studied in the ν ₃ fundamental band of C₂D₂ (≈2440?cm^?1) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. This is the first published spectrum for helium–acetylene. Transitions observed in the region of the C₂D₂ R(0) line are assigned with the help of theoretical results based on an ab initio intermolecular potential, and fitted using a simple Coriolis model. The results indicate that the complex is rather close to the free rotor limit, helping to explain the absence of previous data. Scaled parameters from the model are used to predict a spectrum for He–C₂H₂.     

Anomeric effect plays a major role in the conformational isomerism of fluorinated pnictogen compounds

According to our theoretical studies, the anomeric effect, an stereoelectronic interaction between lone pair and a vicinal antibonding orbital, has shown to contribute decisively for the conformational isomerism of 1‐fluoro‐N,N‐dimethylmethanamine ( 1 ) and of its corresponding P, As and Sb analogues ( 2 – 4 ). C? X bonds in 2 – 4 are larger than in the parent compound 1 , thus providing a LP_X/C? F^* interaction progressively weaker on going from 1 to 4 . However, such hyperconjugation contributed by more than 1.3 kcal mol^?1 for the stabilization of anti conformer in 4 (θ_{LP? X? C? F} = 180°), increasing to 24.1 kcal mol^?1 in 1 . An isodesmic reaction model supported these findings. Copyright © 2008 John Wiley & Sons, Ltd.