1. General introduction 2. The separation of nuclear and electronic motion 3. The electronic hamiltonian 4. Interactions arising from nuclear magnetic and electric moments 5. Angular momentum theory and spherical tensor algebra 6. Electronic and vibrational states 7. Derivation of the effective hamiltonian 8. Molecular beam magnetic and electric resonance 9. Microwave and far-infrared magnetic resonance 10. Pure rotational spectroscopy 11. Double resonance spectroscopy Appendices.

https://physicstoday.scitation.org/doi/pdf/10.1063/1.1878342

Rotational Spectroscopy of Diatomic Molecules

Review of ionic liquids with fluorine-containing anions

The review focusses on the fundamental importance of non-covalent interactions in nature by illustrating specific examples from chemistry, physics and the biosciences. Laser spectroscopic methods and both ab initio and molecular modelling procedures used for the study of non-covalent interactions in molecular clusters are briefly outlined. The role of structure and geometry, stabilization energy, potential and free energy surfaces for molecular clusters is extensively discussed in the light of the most advanced ab initio computational results for the CCSD(T) method, extrapolated to the CBS limit. The most important types of non-covalent complexes are classified and several small and medium size non-covalent systems, including H-bonded and improper H-bonded complexes, nucleic acid base pairs, and peptides and proteins are discussed with some detail. Finally, we evaluate the interpretation of experimental results in comparison with state of the art theoretical models: this is illustrated for phenol...Ar, the benzene dimer and nucleic acid base pairs. A review with 270 references.

/pdf/the-world-of-non-covalent-interactions-2006-43i1v3sdt4.pdf

The World of Non-Covalent Interactions: 2006

Quantum calculations at the MP2/aug-cc-pVDZ level examine complexes pairing HSN with aliphatic amines and phosphines Complexes are cyclic and contain two attractive interactions The first is a SH···N/P H-bond in which the S–H covalent bond contracts and shifts its stretching frequency to the blue, more so for amines than for phosphines The second interaction is different for the amines and phosphines The amines engage in a NH···N H-bond comparable in strength to the aforementioned SH···N interaction In contrast, the second interaction in the phosphine complexes is a direct N···P attraction without an intervening H This interaction is due in part to opposite partial charges on the N and P atoms, as well as covalent forces generated by charge transfer effects

SH···N and SH···P blue-shifting H-bonds and N···P interactions in complexes pairing HSN with amines and phosphines.

Rotational spectroscopy in combination with molecular beams and laser ablation (laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy) has proved to be successful in characterizing the conformers of natural amino acids. The procedure usually followed to assign and identify the different conformers of an amino acid from the rotational spectrum is described through the study of the natural amino acidL-threonine. The solid sample of L-threonine was vaporized by laser pulses, diluted in Ne and supersonically expanded between the mirrors of a Fabry–Perot resonator where it was spectroscopically probed by microwave radiation. The rotational and nuclear quadrupole coupling constants extracted from the analysis of the rotational spectrum are directly compared with those predicted by ab initio methods to achieve the conclusive identification of seven different conformers. A complex hydrogen bonding network arises as a consequence of the polar side chain of threonine.

Seven conformers of L-threonine in the gas phase: a LA-MB-FTMW study.

The solid α-amino acid isoleucine has been vaporized by laser ablation and expanded in a supersonic jet, where the molecular conformations of the isolated molecule were probed using Fourier transform microwave spectroscopy. Two conformers of neutral isoleucine have been detected in gas phase, the most stable being stabilized by an intramolecular hydrogen bond N−H···OC and a cis-COOH arrangement. The higher energy form is stabilized by an intramolecular hydrogen bond N···H−O. The sec-butyl side chain of the amino acid adopts the same configuration in the two observed conformers, with a staggered configuration at Cβ similar to that observed in valine and a trans arrangement of Cα and Cδ. Ab initio calculations at MP2/6-311++G(d,p) level reproduce satisfactorily the experimental results.

Coded amino acids in gas phase: the shape of isoleucine.

4-Hydroxypyrimidine (4HP) has two conformational forms (the hydroxyl hydrogen cis or trans with respect to the adjacent nitrogen), which are in tautomeric equilibrium with two ketonic forms, 4-pyrimidinone (4PO) and 6-pyrimidinone (6PO). We have investigated the free jet absorption millimeterwave spectrum of this system, assigning the rotational spectra of 4HPcis and 4PO; the latter species is more stable by 2.0(9) kJ/mol. No lines corresponding to the trans isomer of 4-hydroxypyridine and to 6PO have been observed.

Gas-phase tautomeric equilibrium of 4-hydroxypyrimidine with its ketonic forms: a free jet millimeterwave spectroscopy study.

Weak hydrogen bonds C–H⋯S and C–H⋯F–C in the thiirane–trifluoromethane dimer

2-Azetidinone–water clusters can be considered as appropriate models for investigating the interaction of the peptide functional group with water. The rotational spectra of 2-azetidinone–(H2O)n (n = 1, 2) complexes have been studied in the 6–18 GHz frequency range using a molecular beam Fourier transform microwave spectrometer. Two different isomers have been observed for the 1 : 1 adduct. The most stable (1 : 1a) is stabilized by two hydrogen bonds O–H⋯OC and N–H⋯O with water closing a ring with the peptide group. For the other conformer (1 : 1b), water is placed on the other side of the carbonyl group stabilized by O–H⋯OC and C–H⋯O hydrogen bonds. In 2-azetidinone–(H2O)2 the water molecules close a ring with the peptide group forming three different hydrogen bonds: O–H⋯OC, O–H⋯O and N–H⋯O. The spectra of the parent and several isotopologues of each cluster have been investigated in order to determine their structures. The hydrogen bond geometries show that the dominant interaction is the O–H⋯OC hydrogen bond, in good agreement with the observed preference of water to interact with this group in proteins. A comparison between the geometries of the different observed adducts shows clearly how cooperative hydrogen bonding plays an important role in the stabilization of these complexes. No detectable structural changes have been observed for 2-azetidinone upon hydration with one water molecule.

Raquel Sánchez

Papers

Coded amino acids in gas phase: the shape of isoleucine.

Gas-phase tautomeric equilibrium of 4-hydroxypyrimidine with its ketonic forms: a free jet millimeterwave spectroscopy study.

Weak hydrogen bonds C–H⋯S and C–H⋯F–C in the thiirane–trifluoromethane dimer

Microsolvation of 2-azetidinone: a model for the peptide group-water interactions.

Tautomeric equilibrium and hydroxyl group internal rotation in 4-hydroxypyridine