High-Resolution Vibrational Spectroscopy of trans-Formic Acid in Solid Parahydrogen

Large amplitude motions within molecules trapped in solid parahydrogen

Molecules of the β-diketone and β-dialdehyde families were trapped in solid parahydrogen (pH2) to investigate the vibrational behavior of systems containing an intramolecular hydrogen bond (IHB). In the simplest β-diketone, acetylacetone (AcAc), H transfer related to the IHB is coupled with methyl torsions. In pH2, the study of nuclear spin conversion (NSC) in methyl groups allows the characterisation of the influence of these large amplitude motions on the vibrational modes. The deuteration of the OH group involved in the IHB has important consequences on the vibrational spectrum of the molecule and evidence of NSC in methyl groups is difficult to obtain. In the chlorine derivative (3-chloroacetylacetone), the H-transfer is no longer coupled with methyl torsion, and NSC has undetectable effects on the IR spectrum. A search of H tunnelling splitting in the IR spectra of β-dialdehydes trapped in pH2 was performed. A few modes of 2-chloromalonaldehyde appear as doublets and were assigned to tunnelling levels. The spectroscopic results related to large amplitude motions are detailed and discussed, highlighting puzzling effects.

Polar solvent fluctuations drive proton transfer in hydrogen bonded complexes of carboxylic acid with pyridines: NMR, IR and ab initio MD study

We study a series of intermolecular hydrogen-bonded 1 : 1 complexes formed by chloroacetic acid with 19 substituted pyridines and one aliphatic amine dissolved in CD₂Cl₂ at low temperature by ¹H and ¹³C NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (O^-H-N⁺) ones, while NMR spectra show the formation of short strong hydrogen bonds in intermediate cases. Analysis of C[double bond, length as m-dash]O stretching and asymmetric CO₂^- stretching bands in FTIR spectra reveal the presence of proton tautomerism. On the basis of these data, we construct the overall proton transfer pathway. In addition to that, we also study by use of ab initio molecular dynamics the complex formed by chloroacetic acid with 2-methylpyridine, surrounded by 71 CD₂Cl₂ molecules, revealing a dual-maximum distribution of hydrogen bond geometries in solution. The analysis of the calculated trajectory shows that the proton jumps between molecular and zwitterionic forms are indeed driven by dipole-dipole solvent-solute interactions, but the primary cause of the jumps is the formation/breaking of weak CHO bonds from solvent molecules to oxygen atoms of the carboxylate group.

Signatures of a quantum diffusion limited hydrogen atom tunneling reaction

We are studying the details of hydrogen atom (H atom) quantum diffusion in parahydrogen quantum solids in an effort to better understand H atom transport and reactivity under these conditions.

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)(-) bonded system

We present a joint experimental and quantum chemical study on the influence of solvent dynamics on the protonation equilibrium in a strongly hydrogen bonded phenol-acetate complex in CD2Cl2. Particular attention is given to the correlation of the proton position distribution with the internal conformation of the complex itself and with fluctuations of the aprotic solvent. Specifically, we have focused on a complex formed by 4-nitrophenol and tetraalkylammonium-acetate in CD2Cl2. Experimentally we have used combined low-temperature (1)H and (13)C NMR and UV-vis spectroscopy and showed that a very strong OHO hydrogen bond is formed with proton tautomerism (PhOH···(-)OAc and PhO(-)···HOAc forms, both strongly hydrogen bonded). Computationally, we have employed ab initio molecular dynamics (70 and 71 solvent molecules, with and without the presence of a counter-cation, respectively). We demonstrate that the relative motion of the counter-cation and the "free" carbonyl group of the acid plays the major role in the OHO bond geometry and causes proton "jumps", i.e. interconversion of PhOH···(-)OAc and PhO(-)···HOAc tautomers. Weak H-bonds between CH(CD) groups of the solvent and the oxygen atom of carbonyl stabilize the PhOH···(-)OAc type of structures. Breaking of CH···O bonds shifts the equilibrium towards PhO(-)···HOAc form.

Matrix isolation spectroscopy and nuclear spin conversion of NH3 and ND3 in solid parahydrogen

We present matrix isolation infrared absorption spectra of NH3 and ND3 trapped in solid parahydrogen (pH2) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC) of NH3 and ND3 in freshly deposited pH2 samples as a tool to assign the sparse vibration-inversion-rotation (VIR) spectra of NH3 in the regions of the ν2, ν4, 2ν4, ν1, and ν3 bands and ND3 in the regions of the ν2, ν4, ν1, and ν3 fundamentals. Partial assignments are also presented for various combination bands of NH3. Detailed analysis of the ν2 bands of NH3 and ND3 indicates that both isotopomers are nearly free rotors; that the vibrational energy is blue-shifted by 1-2%; and that the rotational constants and inversion tunneling splitting are 91-94% and 67-75%, respectively, of the gas-phase values. The line shapes of the VIR absorptions are narrow (0.2-0.4 cm(-1)) for upper states that cannot rotationally relax and broad (>1 cm(-1)) for upper states that can rotationally relax. We report and assign a number of NH3-induced infrared absorption features of the pH2 host near 4150 cm(-1), along with a cooperative transition that involves simultaneous vibrational excitation of a pH2 molecule and rotation-inversion excitation of NH3. The NSCs of NH3 and ND3 were found to follow first-order kinetics with rate constants at 1.8 K of k = 1.88(16) × 10(-3) s(-1) and k = 1.08(8) × 10(-3) s(-1), respectively. These measured rate constants are compared to previous measurements for NH3 in an Ar matrix and with the rate constants measured for other dopant molecules isolated in solid pH2.

Transient H2O Infrared Satellite Peaks Produced in UV Irradiated Formic Acid Doped Solid Parahydrogen

We report newly identified satellite features of the R(0) rovibrational transition of all the fundamental modes of HDO and the ν3 mode of H2O measured via FTIR spectroscopy immediately after the 193 nm in situ photolysis of formic acid (HCOOH and DCOOD) in solid parahydrogen. The intensities of these satellite features decay slowly with a time constant of τ = 121(7) min after photolysis, even when the sample is maintained below 2 K. We propose that the van der Waals complex H···H2O (H···HDO) is the carrier of the satellite peaks and that these metastable complexes are produced after the low-temperature tunneling reaction of the OH (OD) photoproduct with the parahydrogen host.

Theoretical Study of Vibrationally Averaged Dipole Moments for the Ground and Excited C═O Stretching States of trans-Formic Acid

Recent experimental studies of trans-formic acid (FA) in solid para-hydrogen (pH2) highlighted the importance of vibrationally averaged dipole moments for the interpretation of the high-resolution infrared (IR) spectra, in particular for the C═O stretch (ν3) mode. In this report, dipole moments for the ν3 ground (v = 0) and excited (v = 1, 2, 3, and 4) anharmonic vibrational states in trans-FA are investigated using two different approaches: a single mode approximation, where the vibrational states are obtained from the solution of the one-dimensional Schrödinger equation for the harmonic normal coordinate, and a limited vibrational configuration interaction (VCI) approximation. Density functional theory (B3LYP, BPW91) and correlated ab initio (MP2 and CCSD(T)) electronic methods were employed with a number of double- and triple-ζ and correlation consistent basis sets. Both single mode and VCI approaches show comparable agreement with experimental data, which is more dependent on the level of theory used. In particular, the BPW91/cc-pVDZ level appears to perform remarkably well. Effects of solvation of FA in solid state Ar and pH2 matrices were simulated at the BPW91/cc-pVDZ level using a conductor-like polarized continuum model (CPCM). The Ar and pH2 solid-state matrices cause quite a substantial increase in the FA dipole moments. Compared to gas-phase calculations, the CPCM model for pH2 better reproduces the experimental FA spectral shifts caused by interaction with traces of ortho-hydrogen (oH2) species in solid pH2. The validity of the single mode approach is tested against the multidimensional VCI results, suggesting that the isolated (noninteracting) mode approximation is valid up to the third vibrationally excited state (v = 3). Finally, the contribution of the ground anharmonic vibrational states of the remaining modes to the resulting ν3 single mode dipole moments is examined and discussed.

Multiplet splittings and intensities of fine structure components of the Q1(0)H2 + S0(0)N2 transition in a solid parahydrogen matrix

A comprehensive analysis of theoretical multiplet splittings and intensities of the fine structure components of the Q(1)(0)H(2) + S(0)(0)N(2) transition in a solid parahydrogen crystal is presented. The consideration of higher order anisotropic term responsible for splittings is essential to explain the observed splitting of the three components. The pair interaction parameters DeltaB and DeltaC have been determined by comparing the theoretical splittings with the experimental values. The information about the small splittings (approximately 0.1 cm(-1)) due to crystal-field interaction is completely obscured due to fast hopping of v(') = 1, J(') = 0 H(2) vibron. Also, the theoretical expressions are derived for the intensities of the fine structure components of the Q(v(H(2)))(0) + S(v(N(2)))(0) transition and the theoretical results are compared with the experimental findings.