A.C. Legon

Spectroscopic Investigations of Hydrogen Bonding Interactions in the Gas Phase. I. The Determination of the Geometry, Dissociation Energy, Potential Constants and Electric Dipole Moment of the Hydrogen-Bonded Heterodimer HCN

The microwave rotational spectrum of the hydrogen-bonded, linear heterodimer HCN • • • HF has been identified and a number of spectroscopic constants have been measured in a detailed analysis. The spectroscopic constants have been used in a variety of ways in order to evaluate parameters characterizing the potential energy function of the isolated dimer. An investigation of the Stark effect of two rotational transitions of HCN • • • HF has led to an accurate value of the electric dipole moment of the dimer and hence to the enhancement on dimer formation.

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Abstract Rotational transitions of (H 2 O,H 35 Cl) and (H 2 O,H 37 Cl) have been observed and assigned in the frequency range 7–18 GHz. Values of the rotational constants B o + C o and B o − C o , the centrifugal distortion constants D J and D JK and the Cl nuclear quadrupole coupling constants χ aa and χ bb are reported for the vibrational ground state of both species. Arguments based on small spin-spin splittings resolved in some Cl nuclear quadrupole components lead to the conclusion that (H 2 O, HCl) has a pair of equivalent protons and is effectively planar in the zero-point state. Values of r o (O…Cl) have been determined for the planar model in which the HCl molecule forms a hydrogen bond to the oxygen atom of H 2 O.

HF from Its Microwave Rotational Spectrum

Abstract The microwave rotational spectrum of the linear hydrogen cyanide dimer HCN⋯HCN has been identified. The rotational constants, B o , for the three isotopic species HC 14 N⋯HC 14 N, HC 15 N⋯HC 15 N and DCN⋯DCN are respectively 1788.09, 1723.85 and 1698.37 MHz and lead to consistent values for r o (N⋯C), the mean of which is 3.231 A.

Identification and molecular geometry of a weakly bound dimer (H2O, HC1 in the gas phase by rotational spectroscopy

We have assigned the rotational spectrum of H2O • • • HF in vibrationally excited states associated with the hydrogen bond out-of-plane and in-plane bending modes vβ(0) and vβ(i) respectively. This has allowed us to decide whether the equilibrium configuration at the oxygen atom is planar (C2v symmetry) or pyramidal (Cs symmetry) by combining three types of information: vibrational spacing (from relative intensity measurements of vibrational satellites), the dependence of rotational constants on vibrational excitation (from frequency measurements), and the dependence of rotational constants on vibrational excitation (from frequency measurements), and the dependence of the electric dipole moment on the quantum numbers vβ(0) and vβ(1) (from the Stark effect). The vβ(0) = 1 ← 0 separation in D2O • • • DF has also been obtained and used in reaching the conclusion that the equilibrium configuration at oxygen is pyramidal. Under the assumption that the mode vβ(0) is governed by a one-dimensional potential function, a quantitative analysis of all available data leads to the expression V(ϕ)/cm−1=328ϕ4−406ϕ2, where (ϕ) is the angle between the O • • • HF line and the bisector of the angle HOH. This function implies a barrier height of 126 cm-1 or 1.5 kJ mol-1 to inversion of the configuration at oxygen. The equilibrium angle (ϕ) is now available for the series H2O • • • HF ((ϕ) = 46°), (CH2)3O • • • HF ((ϕ) = 54°) and (CH2)2O • • • HF (ϕ = 72°). The variation of (ϕ) along the series is interpreted in terms of the angle between the conventionally viewed non-bonding pairs on the oxygen atom in relation to the angle between the chemical bonds at that atom.

The hydrogen cyanide dimer: Identification and structure from microwave spectroscopy

The microwave rotational spectra of the isotopic species H 2180 • • • H 19F, H 2180 • • • H 19F, HD 160 • •. HF, D2160 • • • DF and D2180 • • • DF of the hydrogen bonded heterodimer formed between water and hydrogen fluoride have been observed and analysed to give rotational constants. Evidence from the effects of nuclear spin statistical weights in the spectra leads to the conclusion that the dimer H 20 • • • HF is either planar with C2V symmetry or pyramidal with Cg symmetry but with a low barrier to inversion of the configuration at oxygen. r0-values of the heavy atom distance 0 * • • F have been derived from the observed rotational constants, the value for H 2160 • • • H 19F being 2.662 Aj. The measured values of electric dipole moment in the vibrational ground state and in the state having v^{0) = 1 are 4.07 and 3.80 D respectively. These values show that excitation of the out-of-plane hydrogen bonding bending mode vP(0) leads to a large decrease in the dipole moment.

Spectroscopic Investigations of Hydrogen Bonding Interactions in the Gas Phase. VII. The Equilibrium Conformation and Out-of-Plane Bending Potential Energy Function of the Hydrogen-Bonded Heterodimer H

The ground state rotational spectra of eight isotopic species of a weakly bound dimer N(1)N(2)u2009⋅⋅⋅u2009HCN(3) formed between molecular nitrogen and hydrogen cyanide have been detected and measured by the technique of pulsed‐nozzle, Fourier‐transform microwave spectroscopy. Rotational constants B0, centrifugal distortion constants DJ, and, where appropriate, nuclear quadrupole coupling constants χn(14N) and χ(D) have been determined. For the five isotopic species containing 14N nuclei the results are: An analysis of the B0 values shows that the equilibrium geometry is linear, or nearly so, with the nuclei in the order shown. The DJ values lead to kσ=2.39 Nu2009m−1 for the intermolecular stretching force constant while the difference χ1(14N)–χ2(14N) is interpreted in terms of a transfer of 0.016 e from N(1) to N(2) when the complex is formed in the zero‐point state.

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The Stark effects in the J=2←1 rotational transitions of the nearly prolate asymmetric rotor H2O⋅⋅⋅HF have been analyzed in the vibrational ground state and in each of the states (1, 0, 0), (0, 1, 0), (0, 0, 1), and (1, 1, 0). The nonmenclature (vβ(o), vβ(i), vσ) refers to the vibrational quantum numbers associated with the out‐of‐plane bending mode vβ(o), the in‐plane bending mode vβ(i) and the stretching mode vσ, respectively, which are the three lowest energy modes of the hydrogen bonded dimer. The electric dipole moment μ for each of these states determined via the Stark effect is as follows:

O ... HF Determined from its Microwave Rotational Spectrum

The ground‐state rotational spectra of three symmetric‐top isotopomers (CH3)3u200914N⋅⋅⋅Hu200979Br, (CH3)3u200914N⋅⋅⋅Hu200981Br, and (CH3)3u200914N⋅⋅⋅Du200979Br of the heterodimer of trimethylamine and hydrogen bromide have been detected by pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The spectroscopic constants B0, DJ, DJK, χ(14N), and χ(Br) have been determined for each of the isotopomers and for (CH3)3u200914N⋅⋅⋅Hu200981Br have the values 1161.6294(2) MHz, 0.148(5) kHz, 7.77(2) kHz, −2.883(7) MHz, and 99.645(7) MHz, respectively. A comparison of the 14N– and 81 Br–nuclear‐quadrupole coupling constants χ(14N) and χ(Br) with those expected on the basis of a hydrogen‐bonded model (CH3)3N⋅⋅⋅HBr and an ion‐pair model (CH3)3NH+⋅⋅⋅Br− leads to the conclusion that in the heterodimer trimethylamine‐hydrogen bromide there is a significant extent of proton transfer from HBr to (CH3)3N. The value of the intermolecular stretching force constant kσ=82(3) Nu2009m−1 determined from DJ is also compared with those expected for the limiting mod...

Spectroscopic Investigations of Hydrogen Bonding Interactions in the Gas Phase. IV. The Heterodimer H

Abstract The microwave spectrum of the heterodimer HCN…HF is reported, the collinearity of the nuclei established, and r 0 (N…F)=2.796 A obtained. From the Stark effect, μ 0 (HCN…HF)=5.59 ± 0.02 D which indicates an enhancement of 0.78 D over the vector sum of the monomer values.

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Abstract The rotational spectra in the vibrational ground states of (H 2 O, HC 14 N) and (H 2 O, HC 15 N) have been assigned in the frequency range 6–19 GHz. Values of rotational constants ( B O , C O ) and centrifugal distortion constants (Δ J , Δ JK ) have been determined for both species, while the 14 N-nuclear quadrupole coupling constants x aa and x bb have been established for the first. Observations concerning additional hyperfine structure arising from H,H nuclear spin-nuclear spin coupling in the H 2 O subunit suggest that (H 2 O,HCN) has a pair of equivalent protons and is effectively planar in the zero-point state. Observed spectroscopic constants are consistent only with the arrangement H 2 O…HCN, with r (O…C) = 3.1387 A.