E. Arunan

Definition of the hydrogen bond (IUPAC Recommendations 2011)

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

The rotational spectrum, structure and dynamics of a benzene dimer

The low J (2 to 7) rotational spectrum of a symmetric‐top benzene dimer has been obtained with a Balle/Flygare Fourier transform microwave spectrometer. Each transition is a symmetrical quartet with two J‐ and K‐dependent tunneling splittings of 30 to 400 kHz. Rotational constants B, DJ, and DJK were determined to be 427.76(2) MHz, 7.2(3) kHz, and 0.869(5) MHz. The dimer is T‐shaped with a benzene c.m. to c.m. distance of 4.96 A.

Low‐J rotational spectra, internal rotation, and structures of several benzene–water dimers

Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted ...

Vibration–rotational Einstein coefficients for HF/DF and HCl/DCl

The experimentally based dipole‐moment functions have been combined with the best Rydberg–Klein–Rees potentials to calculate the vibration–rotational Einstein coefficients for HF, DF, HCl, and DCl. Calculations were done for the Δv=1, 2, and 3 transitions for v≤6 for HF and v≤7 for HCl, which are in the range of the internuclear distance, r, for which the dipole moment functions are valid. The calculations were done for J≤25 for each v level. The higher v levels of HF were investigated using a Pade extrapolation of the experimental dipole function and a recently published ab initio function. Our Δv=1 Einstein coefficients for HF agree closely with those from an earlier experimentally based dipole function and with the new ab initio results for v≤6. Our results for HCl, however, represent a significant improvement over the Einstein coefficients currently in the literature. The isotopic ratio of Einstein coefficients for the Δv=1 transitions, ADX/AHX, were not changed significantly. Also, the changes in the...

The X-C···π (X = F, Cl, Br, CN) carbon bond.

High-level ab initio calculations have been used to study the interactions between the CH3 group of CH3X (X = F, Cl, Br, CN) molecules and π-electrons. These interactions are important because of the abundance of both the CH3 groups and π-electrons in biological systems. Complexes between C2H4/C2H2 and CH3X molecules have been used as model systems. Various theoretical methods such as atoms in molecules theory, reduced density gradient analysis, and natural bond orbital analysis have been used to discern these interactions. These analyses show that the interaction of the π-electrons with the CH3X molecules leads to the formation of X-C···π carbon bonds. Similar complexes with other tetrel molecules, SiH3X and GeH3X, have also been considered.

Rotational spectra and structures of Rg–C6H6–H2O trimers and the Ne–C6H6 dimer (Rg=Ne, Ar, or Kr)

Rotational spectra of Rg–C6H6–H2O isotopomers, where Rg=Ne, Ar, or Kr, have been observed with a Balle–Flygare Fourier transform microwave spectrometer. In these trimers the benzene is sandwiched between the rare gas and H2O. Isotopic substitution and inertial analyses show that the Rg–C6H6 distance in the trimer is reduced by about 0.01 A for Ne, Ar, and Kr compared to the corresponding distance in the Rg–C6H6 dimer. On the other hand, the c.m. (C6H6) to c.m. (H2O) distance in the trimers is increased by only about 0.003 A from its distance in the dimer. Symmetric top spectra of 20Ne–C6H6 and 22Ne–C6H6 were observed as an aid in the comparison. Hyperfine structure (hfs) and substitution analyses with HDO/D2O containing isotopomers reveal that the m=0 and 1 states of H2O in the trimer are virtually unchanged from those in the C6H6–H2O dimer, including essentially free rotation. In addition, analyses are made of the root‐mean square (rms) deviation for the benzene C6 axis from the a axis in the dimers (∼19...

Rotational spectrum of the weakly bonded C6H6–H2S dimer and comparisons to C6H6–H2O dimer

Two symmetric-top,delta J = 1 progressions were observed for the C6H6–H2S dimer using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants for C6H6–H2S are B = 1168.53759(5) MHz, DJ = 1.4424(7) kHz and DJK = 13.634(2) kHz. The other state observed has a smaller B of 1140.580(1) MHz but requires a negative DJ = –13.80(5) kHz and higher order (H) terms to fit the data. Rotational spectra for the isotopomers C6H6–H234S, C6H6–H233S, C6H6–HDS, C6H6–D2S and 13CC5H6–H2S were also obtained. Except for the dimer with HDS, all other isotopomers gave two progressions like the most abundant isotopomer. Analysis of the ground-state data indicates that H2S is located on the C6 axis of the C6H6 with a c.m. (C6H6)–S distance of 3.818 A. The angle between the a axis of the dimer and the C2v axis of the H2S is determined to be 28.5°. The C6 axis of C6H6 is nearly coincident with a axis of the dimer. Stark measurements of the two states led to dipole moments of 1.14(2) D for the ground state and 0.96(6) D for the other state. A third progression was observed for C6H6–H2S which appear to have K0 lines split by several MHz, suggesting a nonzero projection of the internal rotation angular momentum of H2S on the dimer a axis. The observation of three different states suggests that the H2S is rotating in a nearly spherical potential leading to three internal rotor states, two of which have Mj = 0 and one having Mj = ±1,Mj being the projection of internal rotational angular momentum on to the a axis of the dimer. The nuclear quadrupole hyperfine constant of the 33S nucleus in the dimer is determined for the two symmetric-top progressions and they are –17.11 MHz for the ground state and –8.45 MHz for the other state, consistent with the assignment to two different internal-rotor states. The 17O quadrupole coupling constant for the two states of C6H6–H2O were measured for comparison and it turned out to be nearly the same in the ground and excited internal rotor state, –1.89 and –1.99 MHz, respectively. The rotational spectrum of the C6H6–H2S complex is very different from that of the C6H6–H2O complex. Model potential calculations predict small barriers of 227, 121, and 356 cm–1 for rotation about a, b and c axes of H2S, respectively, giving quantitative support for the experimental conclusion that H2S is effectively freely rotating in a nearly spherical potential. For the C6H6–H2O complex, the corresponding barriers are 365, 298 and 590 cm–1.

Is there a hydrogen bond radius? Evidence from microwave spectroscopy, neutron scattering and X-ray diffraction results

Intermolecular distances in D–H⋯A hydrogen bonded systems have usually been interpreted in terms of the van der Waals radii of D and A. In this work, X-ray and neutron diffraction data from the Cambridge Crystal Structure Database (CSD) and the electrostatic potential of A, have been used to define hydrogen bond radii for OH, NH and CH groups. For OH, X-ray and neutron diffraction both give comparable results, validating the X-ray data for defining a hydrogen bond radius. The hydrogen bond radii determined for CCH and OH groups from CSD analysis are comparable to those determined from the gas phase rotational spectroscopic data for HCCH and H2O complexes. For NH as a proton donor, gas phase structural data are scarce and a hydrogen bond radius has been determined by using X-ray diffraction data only. For the CH group, the histogram of hydrogen bond distances shows a peak recognizable as a hydrogen bond only if it is acidic such as CCl3H, OCH (aldehydic) or CCH (acetylenic). The hydrogen bond radii for OH, NH and acidic CH groups are 0.60 ± 0.15, 0.76 ± 0.15 and 1.10 ± 0.20 A, respectively. For C–CH3 and CH2CH3, though a peak in the histogram of distances is not found, the distribution of hydrogen bond angles unambiguously shows that the preferred geometry is linear. It appears that a CH group without any electronegative substituents could have a radius larger than 1.2 A when involved in hydrogen bonding.

Pulsed Nozzle Fourier Transform Microwave Spectrometer: Advances and Applications

Abstract The pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer was developed by Balle and Flygare [A new method for observing the rotational spectra of weak molecular complexes: KrHCl. J. Chem. Phys. 1979, 71 (6), 2723–2724 and 1980, 72 (2), 922–932] in 1979. The design, fabrication, and operation of this spectrometer are complicated and it has largely remained a research laboratory tool till now, though a portable spectrometer for routine analytical applications has been developed at the National Institute for Standards and Technology [Suenram, R.D.; Grabow, J.‐U.; Zuban, A.; Leonov, I. A portable pulsed‐molecular‐beam Fourier‐transform microwave spectrometer designed for chemical analysis. Rev. Sci. Instrum. 1999, 70 (4), 2127–2135]. However, the potential for extracting fundamental information about any chemical species, such as, molecules, radicals, ions, or weakly bound complexes between any of them including atoms, has been quite significant. It is evident from the fact that more than 25 laboratories around the globe have built this spectrometer, some in the recent past. Contributions from all these laboratories have widened the horizon of PNFTMW spectrometers applications. This review summarizes the advances in design and the recent applications of this spectrometer. We also define an electrophore, as an atom/molecule that generates an electric dipole moment by forming a weakly bound complex with a species having zero electric dipole moment. The electrophore, thereby, enables structural determination using rotational spectroscopy, as in the case of Ar2–Ne, with Ne as the electrophore. Also, it can introduce a dipole moment about a principal axis where none existed before, such as in Ar–(H2O)2, enabling the observation of pure rotational transitions for several tunneling states.