Raman studies of solid hydrogen cyanide (HCN,DCN) and of HCN argon matrices

Raman spectra of single crystal-like oriented solid HCN and DCN, as well as of HCN molecules and clusters in argon matrices, have been recorded and analysed with respect to both so far uninterpreted spectral features reported in the literature and to the applicability of Raman experiments for studying molecular clusters in inert gas matrices. The main results obtained are: (i) splitting of both the intramolecular bending modes and librations of solid HCN has to be interpreted in terms of TO/LO splitting rather than site group splitting; (ii) the nature of the phase transition at 170 K reported as tI6 to oI6 must be doubted; and (iii) the previous results of HCN(DCN)/Ar matrices obtained by IR studies are confirmed.     

Influence of rotation and isotope effects on the dynamics of the N((2)D)+H(2) reactive system and of its deuterated variants

Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.     

1,1-Elimination of HCN from propionitrile under electron-impact. Hydrogen migration in the molecular ion

The dominant primary reactions of propionitrile under electron-impact effect loss of HCN and loss of H·. Deuterium labeling shows that the hydrogen atom lost as HCN comes chiefly from C-2, but that lost as a free atom comes chiefly from C-3. Both reactions are probably preceded by a 1,2 hydrogen migration to yield an allylic-like molecular ion, \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm CH}_{\rm 3}{\rm C}^+ {\rm HCH = N}^{\rm .} \leftrightarrow {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm .} {\rm HCH = N}} \right]^+ $\end{document}.     

Intramolecular catalysis in the mass spectrometer: Mechanisms for loss of methanol from methyl esters upon electron-impact

A review of the literature indicates compelling evidence that: (1) loss of ROH from esters requires protonation of the alkoxy oxygen; (2) the (symmetry forbidden) [1,3] hydrogen migration from protonated carbonyl to alkoxy oxygen does not occur in the mass spectra of esters; (3) hydrogen abstraction in esters occurs almost exclusively to the carbonyl oxygen. Mechanisms are proposed which account for all examples of ROH loss from esters. Alkanol loss from molecular ions in esters requires the presence of a second functional group to act as an intramolecular catalyst, either as a general acid in transferring a proton to the alkoxy oxygen, or as a general base in assisting the [1,3] carbonyl oxygen to alkoxy oxygen proton transfer. Loss of ROH from fragment ions requires proton transfer from an atom α to the positive charge to the alkoxy oxygen. These mechanisms are generalized to include a wide class of bifunctional esters and a selection of natural products.     

Geometric H/D isotope effects and cooperativity of the hydrogen bonds in porphycene.

We investigate the primary, secondary, and vicinal hydrogen/deuterium (H/D) isotope effects on the geometry of the two intramolecular hydrogen bonds in porphycene. Multidimensional potential energy surfaces describing the anharmonic motion in the vicinity of the trans isomer are calculated for the different symmetric (HH/DD) and asymmetric (HD) isotopomers. From the solution of the nuclear Schr?dinger equation the ground-state wavefunction is obtained, which is further used to determine the quantum corrections to the classical equilibrium geometries of the hydrogen bonds and thus the geometric isotope effects. In particular, it is found that the hydrogen bonds are cooperative, that is, both expand simultaneously even in the case of an asymmetric isotopic substitution. The theoretical predictions compare favorably with NMR chemical-shift data.     

Deuterium kinetic isotope effects on the gas-phase reactions of C2H with H2(D2) and CH4(CD4)

Kinetics of the ethynyl (C(2)H) radical reactions with H(2), D(2), CH(4) and CD(4) was studied over the temperature range of 295-396 K by a pulsed laser photolysis/chemiluminescence technique. The C(2)H radicals were generated by ArF excimer-laser photolysis of C(2)H(2) or CF(3)C(2)H and were monitored by the chemiluminescence of CH(A(2)Δ) produced by their reaction with O(2) or O((3)P). The measured absolute rate constants for H(2) and CH(4) agreed well with the available literature data. The primary kinetic isotope effects (KIEs) were determined to be k(H(2))/k(D(2)) = 2.48 ± 0.14 and k(CH(4))/k(CD(4)) = 2.45 ± 0.16 at room temperature. Both of the KIEs increased as the temperature was lowered. The KIEs were analyzed by using the variational transition state theory with semiclassical small-curvature tunneling corrections. With anharmonic corrections on the loose transitional vibrational modes of the transition states, the theoretical predictions satisfactorily reproduced the experimental KIEs for both C(2)H + H(2)(D(2)) and C(2)H + CH(4)(CD(4)) reactions.     

Distinguishing the formation of C2D+4 ions from C2D+6 (by D2 loss) and from C2D5H+ (by HD loss) in a Reflectron spectrometer

The D₂ loss from C₂D⁺₆ ions and the HD loss from C₂D₅H⁺ ions has been investigated in a photoelectron photoion coincidence experiment employing a reflecting ion time of a flight mass spectrometer (Reflectron). The experiment is able to distinguish the metastable formation of C₂D⁺₄ ions (m/z = 32) from C₂D⁺₆ ions by D₂ loss and from C₂D₅H⁺ ions by HD loss simultaneously in a mixture of deuterated ethanes. The breakdown curves of the title reactions are presented and compared to the H₂ loss from C₂H⁺₆ ions. The HD loss from C₂D₅H⁺ is shifted by 67 meV and the D₂ loss from C₂D⁺₆ is shifted by 108 meV with respect to the H₂ loss from C₂H⁺₆. This shift reflects a strong kinetic isotope effect which is most likely due to tunneling of H/D atoms through a barrier.     

Quantitative 2H NMR spectroscopy. 1. Thermodynamic H/D isotope effects in N,N-dimethylformamide and cyclopentadiene molecules

A new method for quantification of the relative distribution of deuterium in molecules is proposed. The technique is based on the lineshape analysis in the ²H NMR spectra obtained at the natural abundance level of deuterium with allowance for inhomogeneity of the magnetic field. The equilibrium thermodynamic H/D isotope effects for hindered rotation about the C—N bond in the N,N-dimethylformamide molecule and for prototropic exchange in the cyclopentadiene molecule were determined. The results obtained agree with those of DFT calculations of the vibrational energies.     

Primary and secondary kinetic isotope effects in proton (H(+)/D(+)) and chloronium ion ((35)Cl(+)/(37)Cl(+)) affinities.

The Cooks' kinetic method and tandem-in-space pentaquadrupole QqQqQ mass spectrometry were used to measure primary and secondary kinetic isotope effects (KIEs) in H(+) and Cl(+) (X(+)) affinity for a series of A/A(') isotopomeric pairs. Gaseous, isotopomeric, and loosely bound dimers [A...X(+)...A(')] were formed in combinations in which X = H(+), D(+), (35)Cl(+) or (37)Cl(+) and A/A(') = acetonitrile/acetonitrile - d(3), acetonitrile/acetonitrile-(15)N, acetonitrile-d(3)/acetonitrile-(15)N, acetone/acetone-d(6), acetone/acetone-(18)O, acetone-d(6)/acetone-(18)O, pyridine/pyridine-d(5), pyridine/pyridine-(15)N, pyridine-d(5)/pyridine-(15)N, or 3-((35)Cl)chloropyridine/3-((37)Cl)chloropyridine. Under nearly the same experimental conditions, the dimers were mass-selected and then dissociated by low-energy collisions with argon, yielding AX(+) and A(')X(+) as the fragment ions. KIEs were measured from the changes in ion affinities of the neutrals (DeltaX(+)) as estimated by the AX(+)/A(')X(+) abundance ratios. Using [A...H(+)(D(+))...A(')] and [A...(35)Cl(+)((37)Cl(+))...A(')] dimers and by comparing their extent of dissociation under nearly identical collision-induced dissociation conditions, the kinetic method was also applied, for the first time, to measure primary KIEs of the central ion as well as their influence on secondary KIEs. Becke3LYP/6-311++G(2df,2p) calculations were found to provide Delta(DeltaZPE)s for the competitive dissociation reactions that accurately predict the nature (normal or inverse) of the measured KIEs.     

Dynamics of formation of CN(B2Σ+) fragment from HCN and DCN in an argon afterglow

The dissociative excitation of HCN and DCN producing CN(B²Σ⁺) in collision with Ar(³P_0,2) was investigated in a flowing afterglow. The Δν = 0, ?1, and ?2 sequences of the CN(BX) violet emission were analyzed by computer simulation, and the vibrational and rotational distributions of the CN(B²Σ⁺) fragment were obtained. Possible reaction pathways were studied on the basis of a linear surprisal analysis of the observed distributions and their isotope effects.     

H/D isotope exchange in 9-R-sym-octahydro-10-oxonia(chalcogenonia)anthracene perchlorates

The method of controlled H/D isotope exchange in acid media has been used to study the effect of the heteroatom and the substituents in 9-R-sym-octahydro-10-oxonia(chalcogenonia)anthracene perchlorates on the mobility of hydrogen atoms in the -methylene units of alicyclic rings condensed with heteroaromatic rings. It has been established that in a group of aryl-substituted salts the mobility of the hydrogen atoms increases in the series S⁺++, which corresponds to a decrease in thermodynamic stability of the chalcogenopyrylium cations. In a group of unsubstituted salts (R=H) the order of reactivity of the methylene groups is O⁺++ and corresponds to the increasing orbital electrophilicity and complexing ability of these cations.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 463–467, April, 1993.     

Reactions of C(1D) with H2 and its deuterated isotopomers, a wave packet study

Using a Chebyshev wave packet method, total and state-resolved reaction probabilities (J=0) were calculated for the reactions of C(1D) with various hydrogen isotopomers (H2, D2, and HD, nu i=0, j i=0) on a recent ab initio potential energy surface. For all the isotopic variants, it was found that the initial state specified reaction probabilities have no energy threshold and are strongly oscillatory, indicative of the involvement of long-lived resonances in this barrierless reaction. The J=0 product vibrational and rotational distributions for all three isotopic reactions, and the CH/CD branching ratio for the C+HD reaction, show strong dependence on the collision energy, further underscoring the important role played by the resonances. The generally decaying vibrational distributions and highly excited rotational distributions, which corroborate an insertion mechanism, and the dominance of the CD+H channel in the C+HD reaction are consistent with existing experimental observations. Initial state specified integral cross sections and rate constants were estimated using a capture model. The estimated rate constants were found to be close and in the order kHD>kH2>kD2. Finally, a method to calculate branching ratio in the C+HD reaction is proposed.     

The electron-impact induced loss of CHNO from 1,3-bis-(3-m-tolyl-1,2,4-oxadiazol-5-yl)hexafluoropropane

High resolution mass measurement established that the m/e 425 peak in the mass spectrum of 1,3-bis-(3-m-tolyl-1, 2,4-oxadiazol-5-yl)hexafluoropropane corresponds to [M ? CHNO]. This peak is absent from the mass spectrum of the para isomer.     

Theoretical studies of magnetic shielding in HCN and (HCN)2

The magnetic shielding tensors of the ¹H, ¹³C and ¹⁵N nuclei in HCN dimer are calculated with GIAO SCF perturbation theory. The effects of electronic charge distribution and intermolecular interaction are studied. We showed that the shielding of the three nuclei cannot be interpreted by simple correlation to their electron population. A careful analysis of neighbouring atom contribution is required.     

Production yields of H(D) atoms in the reactions of N(2)(A (3)Sigma(u) (+)) with C(2)H(2), C(2)H(4), and their deuterated variants

The production yields of H(D) atoms in the reactions of N(2)(A (3)Sigma(u) (+)) with C(2)H(2), C(2)H(4), and their deuterated variants were determined. N(2)(A (3)Sigma(u) (+)) was produced by excitation transfer between Xe(6s[32](1)) and ground-state N(2) followed by collisional relaxation. Xe(6s[32](1)) was produced by two-photon laser excitation of Xe(6p[12](0)) followed by concomitant amplified spontaneous emission. H(D) atoms were detected by using vacuum-ultraviolet laser-induced fluorescence (LIF). The H(D)-atom yields were evaluated from the LIF intensities and the overall rate constants for the quenching, which were determined from the temporal profiles of the NO tracer emission. The absolute yields were evaluated by assuming that the yield for NH(3)(ND(3)) is 0.9. Although no HD isotope effects were observed in the overall rate constants, there were isotope effects in the H(D)-atom yields. The H-atom yields for C(2)H(2) and C(2)H(4) were 0.52 and 0.30, respectively, while the D-atom yields for C(2)D(2) and C(2)D(4) were 0.33 and 0.13, respectively. The presence of isotope effects in yields suggests that H(2)(D(2)) molecular elimination processes are competing and that molecular elimination is more dominant in deuterated species than in hydrides.     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

Experimental and quantum-chemical determination of the (2)H quadrupole coupling tensor in deuterated benzenes

Deuterium Quadrupole Coupling Constant (DQCC) in benzene was determined both experimentally by Nuclear Magnetic Resonance spectroscopy in Liquid Crystalline solutions (LC NMR) and theoretically by ab initio electronic structure calculations. DQCCs were measured for benzene-d(1) and 1,3,5-benzene-d(3) using several different liquid crystalline solvents and taking vibrational and deformational corrections into account in the analysis of experimental dipolar couplings, used to determine the orientational order parameter of the dissolved benzene. The experimental DQCC results for the isotopomers benzene-d(1) and 1,3,5-benzene-d(3) are found to be 187.7 kHz and 187.3 kHz, respectively, which are essentially equal within the experimental accuracy (+/-0.4 kHz). Theoretical results were obtained at different C-D bond lengths, and by applying corrections for electron correlation and rovibrational motion on top of large-basis-set Hartree-Fock results. The computations give a consistent DQCC of ca. 189 kHz for three different isotopomers; benzene-d(1), 1,3,5-benzene-d(3), and benzene-d(6), revealing that isotope effects are not detectable within the present experimental accuracy. Calculations carried out using a continuum solvation model to account for intermolecular interaction effects result in very small changes as compared to the data obtained in vacuo. The comparison of theoretical and experimental results points out the selection of the underlying molecular geometry as the most likely source of the remaining discrepancy of less than 2 kHz. Such an agreement between the calculated and the experimental DQCC results can only be achieved if rovibrational effects are considered on one hand in the experimental direct dipolar coupling data, and on the other hand in the theoretical property calculation, as is done presently.     

1H/2H NMR studies of geometric H/D isotope effects on the coupled hydrogen bonds in porphycene derivatives

The 1H and 2H NMR spectra of porphycene (1), 2,3,6,7,12,13,16,17-octaethylporphycene (2), 2,7,12,17-tetra-n-propylporphycene (3), and 2,7,12,17-tetra-(tert-butyl)-3,6-13,16-dibenzo[cde;mno]porphycene (4) partially deuterated in the mobile proton sites are reported. These compounds exhibit two intramolecular NHN hydrogen bonds of increasing strength representing models of the concerted HH transfer in the parent compound, porphycene. The 1H chemical shifts of the mobile protons are correlated with the difference of the energies of the amino- and imino-N1s orbitals reported by Ghosh A.; Moulder J.; Br?ring M.; Vogel E. Angew. Chem., Int. Ed. 2001, 113, 445-448. The chemical shifts of 4 indicate a reduced contribution of the aromatic ring current as compared to the other compounds which is associated to the nonplanarity of this molecule. The primary H/D isotope effects on the chemical shifts give information about the primary, secondary, and vicinal geometric isotope effects of the two inner hydrogen bonds of porphycenes. The vicinal effects indicate a cooperative coupling of the two hydrogen bonds which may favor a concerted double proton-transfer mechanism.     

Reactive scattering of H and D atoms. II. isotope effect on the angular distribution of H,D + NO2

The chemical reactions of hydrogen and deuterium atoms with nitrogendioxide (NO₂) have been studied with crossed molecular beams. Angular and velocity distributions have been measured. The angular distribution peaks at small angles and shows a pronounced isotope effect; only 2̃4% of the available energy goes into translation. Al-though four very deep potential wells (2.1 eV) exist in the potential hypersurface no statistical complex is formed. The reaction can probably be described as intermediate between direct and complex scattering.     

The ultraviolet photodissociation of DCl: H/D isotope effects on the Cl(P) atom spin–orbit branching

The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV absorption band (A¹Π₁←X¹Σ⁺) has been investigated at five wavelengths in the range 200–220 nm. Ground state Cl(²P_3/2) and spin–orbit excited Cl^*(²P_1/2) photofragments were monitored using (2+1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant H/D isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.