a b s t r a c tAs the archetype of water hydrogen bonding, the water dimer has been studied extensively by both theory and experiment for nearly seven decades. In this article, we present a detailed chronological review of the experimental dimer studies and the insights into the complex nature of water and hydrogen bonding gained from them. A subsequent letter will review the corresponding theoretical advances.

Molecular simulations from small molecules to large bio-macromolecules and polymer systems are routinely used to simulate thermodynamics properties of interests by molecular mechanics-based potentials. In a recent paper, via three different semi-empirical methods, we reported quantum singularities in molecular mechanics torsion potentials as signature of chemical bond break-up process revealed under experimental X-ray as broken chemical moieties. In this present work, applying rst principle methods of Hartree-Fock, Density Functional as well as Moller-Plesset techniques we have recon rmed the previous general predictions of singularities in the torsion potential for the case of water dimer that connects two water monomers by weak hydrogen bond. Due to quantum nature of chemical bond breaking process leading to break-point conditions in otherwise connected molecular topology aided by molecular mechanics based potentials, the singularities are also suggestive of large forces as onset in the bond-breaking process. We have presented the details of these novel interesting ndings in this paper. These results of quantum singularities can have signi cant impacts to improve current force elds and can open up new areas we de ne as "Fracture Molecular Mechanics" or "Fracture Force Field" in overlap regions of molecular and quantum mechanics based approaches to explore and account for chemical bond-breaking mechanisms in molecular simulation techniques.

Blue-shifting C-H...O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl(4) solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (nu(C-H)) in the said three complexes are +1, +2, and +5 cm(-1), respectively, and the complexation results in enhancement of the nu(C-H) transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (nu(C=O)) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The nu(C=O) bands in the three complexes are red-shifted by 8, 19, and 6 cm(-1), and apparently have no correlation with the respective blue shifts of the nu(C-H) bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and C=O (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) --> sigma*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the nu(C-H) transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.

In 1:1 CH...O hydrogen bonded complexes between haloforms and ethers, a correlation of the spectral shifts of nu(C-H) bands (Deltanu(C-H)) of the donors (haloforms) with C-O-C angular strain of the acceptors (ethers) is investigated by the electronic structure theory method at the MP2/6-311++G** level. The calculation predicts that the three-member cyclic ether (oxirane) that has the smallest C-O-C angle induces a much larger blue shifting effect on nu(C-H) transition of fluoroform compared with that by the open chain analogue, dimethyl ether. The natural bond orbital (NBO) analysis reveals that the effect originates because of higher "s" character in the hybrid lone electron pair orbital of the oxygen atom of the former, which is responsible for a smaller contribution to n(O) --> sigma*(C-H) hyperconjugation interaction energy between the donor-acceptor molecules. The optimized structures of the two complexes are largely different with respect to the intermolecular orientational parameters at the hydrogen bonding sites, and similar behavior is also predicted for the two chloroform complexes. Partial optimizations on a series of structures show that the total binding energy of the complexes are insensitive with respect to those geometric parameters. However, the Deltanu(C-H), hyperconjugation interaction energies and hybridization of the carbon-centric bonding orbital of the C-H bond are sensitive with respect to those parameters. The predicted Deltanu(C-H) of each complex is analyzed with respect to the IR spectral shift measured by van der Veken and coworkers in cryosolutions of inert gases. The disagreement found between the measured and calculated IR shifts is interpreted to be the outcome of deformation of the complex geometries along shallow binding potential energy surfaces owing to solvation in the liquefied inert gases.

DFT (U)B3LYP/cc-pVTZ and (U)M06-2X/cc-pVTZ and multireference CASSCF/cc-pVTZ level of theories have been used to investigate the electronic structure of isomeric dehydrooxazole (1b-d, 3 isomers), dehydrothiazole (2b-d, 3 isomers) and dehydroimidazole (3a-d, 4 isomers) radicals. The ground state electronic structure of each radical isomer has been confirmed by predicting their doublet excited state structure and calculating the adiabatic energy difference. The stability order of the individual isomeric radicals has been estimated through the comparison of absolute energies. A hypothetical isodesmic reaction has been utilized to calculate radical stabilization energies (RSEs). The results show N-dehydro imidazole 3a as the thermodynamically most stable radical. In order to understand the structural and stability aspects of the radicals and interactions between the radical electron and the electron lone pairs, we have analysed spin densities and hybridisation changes and also performed MCSCF calculations. The reason for the higher stability of 3a has been attributed to the attainment of a π-character and subsequent delocalization, whereas, all other carbon centred radicals are found to be localized σ-radicals. Furthermore, the kinetic stability of the radicals has been investigated through unimolecular decomposition channels. All the studies showed a weak to strong coupling between the N3 and the radical centre depending on the location of the radical centre. NBO analysis suggests that through space coupling between N3 and σ* of the radical centre leads to a stabilising effect when the radical centre is adjacent to N3, whereas such interactions are absent when the radical centre is away from it. However, only a weak coupling is observed between the radical centre and X1 (O/S/NH). Particularly the interaction strength has the following trend: S < O < N-H. Indeed S, O and N-H show a stabilising effect through bond interactions with the antibonding orbitals of the alternate bonds on either side.