Infrared-Vacuum Ultraviolet Spectroscopic and Theoretical Study of Neutral Methylamine Dimer

Growth mechanism of aromatic prebiotic molecules: insights from different processes of ion-molecule reactions in benzonitrile-ammonia and benzonitrile-methylamine clusters

Benzonitrile (BN, C6H5CN) has been detected in the cold molecular cloud Taurus molecular cloud-1 (TMC-1) in 2018, which is suggested to be a precursor in the formation of more complex nitrogen-containing aromatic interstellar compounds. In this study, we utilized mass-selected infrared (IR) photodissociation spectroscopy and quantum chemical calculations to investigate the structures and gaseous ion-molecule reactions of benzonitrile-ammonia (BN-NH3) and benzonitrile-methylamine (BN-MA) clusters. The spectral observations indicate that the cyclic hydrogen bonding structure predominates in both neutral clusters. After VUV (118 nm) single-photon ionization, a new C-N covalent bond formed between BN and NH3 in the (BN-NH3)+ cluster. However, proton sharing constitutes the primary structure of the (BN-MA)+ cluster. The two nitrogen-containing interstellar molecules react with BN to yield distinct products due to difference in charge distribution and molecular polarity in the ionized clusters. The reactions of BN with other molecules contribute to our understanding of the growth mechanisms of complex interstellar molecules.

Structures and Anharmonic Analyses of the O-H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H2O)n, and (BA)2(H2O)n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures

The IR spectra of benzoic acid (BA), (BA)(H2O)n and (BA)2(H2O)n (n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm-1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H2O) and (BA)(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA)m(H2O)n and their ring-deuterated isotopologues (BA-d5)m(H2O)n for specific values of m and n. The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H2O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.

Structures of Pyridine-Water Clusters Studied with Infrared-Vacuum Ultraviolet Spectroscopy

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd)_m(H₂O)_n, with m, n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)_m(H₂O)_n⁺ by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)_m(H₂O)_n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)₂⁺ cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)₂(H₂O)_n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)₂(H₂O)_n clusters had the characteristic structure in which H₂O or (H₂O)₂ forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)₂, and an additional H₂O molecule(s) extends the H-bonded network. The π-stacked (Pyd)₂(H₂O)_n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd)_m(H₂O)_n was found to be less pronounced in the case of (Pyd)_m(NH₃)_n studied previously.

Understanding Fermi resonances in the complex vibrational spectra of the methyl groups in methylamines

Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured in three experimental set-ups to capture their complex spectral features as a result of bend/umbrella-stretch Fermi resonance (FR). Multiple bands were observed between 2800 and 3000 cm-1 corresponding to the methyl groups for MMA and DMA. On the other hand, the corresponding spectrum of TMA is relatively simple, exhibiting only four prominent bands in the same frequency window, even though TMA has a larger number of methyl groups. The discrete variable representation (DVR) based ab initio anharmonic algorithm with potential energy surface (PES) at CCSD/aug-cc-pVDZ quality is able to capture all the experimentally observed spectral features across all three amines, and the constructed vibrational Hamiltonian was used to analyze the couplings that give rise to the observed FR patterns. It was observed that the vibrational coupling among CH stretch modes on different methyl groups is weak (less than 2 cm-1) and stronger vibrational coupling is found to localize within a methyl group. In MMA and DMA, the complex feature between 2850 and 2950 cm-1 is a consequence of closely packed overtone states that gain intensities by mixing with the stretching modes. The simplification of the spectral pattern of TMA can be understood by the red-shift of the symmetric CH3 stretching modes by about 80 cm-1 relative to MMA, which causes the symmetric CH3 stretch to shift outside the FR window.

IR-VUV spectroscopy of pyridine dimers, trimers and pyridine-ammonia complexes in a supersonic jet

The infrared spectra of the C-H stretching vibrations of (pyridine)m, m = 1-3, and the N-H stretching vibrations of (pyridine)m-(NH3)n, m = 1, 2; n = 1-4, complexes were investigated by infrared (IR)-vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP0) of the pyridine monomer was determined to be 74 546 cm-1 (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C-H stretching region at 3000-3200 cm-1. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C-H stretching fundamentals. For (pyridine)2 and (pyridine)3, most C-H bands are blue-shifted by 3-5 cm-1 from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)2, while (pyridine)3 prefers the structures stabilized by dipole-dipole and C-Hπ interactions. For the (pyridine)m-(NH3)n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N-H stretching vibrations of the complexes showed four main bands in the 3200-3450 cm-1 region. These features are very similar to those of (NH3)n complexes, and the bands are assigned to the anti-symmetric N-H stretching band (ν3), the symmetric N-H stretching (ν1) band, and the first overtone bands of the N-H bending vibrations (2ν4). The anharmonic calculations including the Fermi-resonance between ν1 and 2ν4 well reproduced the observed spectra.

Infrared spectroscopic observation of the McLafferty rearrangement in ionized 2-pentanone

The isomerization mechanism of ionized 2-pentanone is investigated by infrared predissociation spectroscopy and theoretical calculations. The observation of OH stretch demonstrates its enolization through the McLafferty rearrangement.

Vibrational spectra of small methylamine clusters accessed by an ab initio anharmonic approach

Anharmonic vibrational calculations on Methylamine (MMA) clusters suggest that the origin of the complexity between 2800 and 3000 cm^–1 is caused by Fermi resonance (FR) between the stretching and bending overtones of the CH₃ group. In trimer and tetramer, FR also causes complex spectra pattern in the NH₂ group.

Infrared photodissociation spectroscopy of cold cationic trimethylamine complexes

Infrared spectroscopic studies reveal the general trends in the stepwise growth motif of trimethylamine (TMA)_n⁺ complexes.

Hydrogen bond induced enhancement of Fermi resonances in N–H⋯N hydrogen bonded complexes of anilines

Enhancement of Fermi resonance intensities due to the formation of N–H⋯N hydrogen bonding of anilines with alkyl amines is analyzed using a two-state deperturbation model.