Microwave spectrum, structure, dipole moment, and deuterium nuclear quadrupole coupling constants of the acetylene–sulfur dioxide van der Waals complex

Thirty‐three a‐ and c‐dipole transitions of the acetylene–SO2 van der Waals complex have been observed by Fourier transform microwave spectroscopy and fit to rotational constants A=7176.804(2) MHz, B=2234.962(1) MHz, C=1796.160(1) MHz. The complex has Cs symmetry with the C2H2 and SO2 moieties both straddling an a–c symmetry plane (i.e., only the S atom lies in the plane). The two subunits are separated by a distance Rcm=3.430(1) Å and the C2 axis of the SO2 is tilted 14.1(1)° from perpendicular to the Rcm vector, with the S atom closer to the C2H2. The dipole moment of the complex is 1.683(5) D. The deuterium nuclear quadrupole hyperfine structure was resolved for several transitions in both C2HD⋅SO2 and C2D2⋅SO2. A lower limit for the barrier to internal rotation of the C2H2 was estimated to be 150 cm−1 from the absence of tunneling splittings. The binding energy was estimated by the pseudo‐diatomic model as 2.1 kcal/mol. A distributed multipole analysis was investigated to rationalize the structure and binding of the complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69970/2/JCPSA6-94-11-6947-1.pd

Watson–Crick and Sugar-Edge Base Pairing of Cytosine in the Gas Phase: UV and Infrared Spectra of Cytosine·2-Pyridone

While keto-amino cytosine is the dominant species in aqueous solution, spectroscopic studies in molecular beams and in noble gas matrices show that other cytosine tautomers prevail in apolar environments. Each of these offers two or three H-bonding sites (Watson–Crick, wobble, sugar-edge). The mass- and isomer-specific S1 ← S0 vibronic spectra of cytosine·2-pyridone (Cyt·2PY) and 1-methylcytosine·2PY are measured using UV laser resonant two-photon ionization (R2PI), UV/UV depletion, and IR depletion spectroscopy. The UV spectra of the Watson–Crick and sugar-edge isomers of Cyt·2PY are separated using UV/UV spectral hole-burning. Five different isomers of Cyt·2PY are observed in a supersonic beam. We show that the Watson–Crick and sugar-edge dimers of keto-amino cytosine with 2PY are the most abundant in the beam, although keto-amino-cytosine is only the third most abundant tautomer in the gas phase. We identify the different isomers by combining three different diagnostic tools: (1) methylation of the cytosine N1–H group prevents formation of both the sugar-edge and wobble isomers and gives the Watson–Crick isomer exclusively. (2) The calculated ground state binding and dissociation energies, relative gas-phase abundances, excitation and the ionization energies are in agreement with the assignment of the dominant Cyt·2PY isomers to the Watson–Crick and sugar-edge complexes of keto-amino cytosine. (3) The comparison of calculated ground state vibrational frequencies to the experimental IR spectra in the carbonyl stretch and NH/OH/CH stretch ranges strengthen this identification

Effects of Charge and Substituent on the S∙∙∙N Chalcogen Bond

Neutral complexes containing a S···N chalcogen bond are compared with similar systems in which a positive charge has been added to the S-containing electron acceptor, using high-level ab initio calculations. The effects on both XS···N and XS+···N bonds are evaluated for a range of different substituents X = CH3, CF3, NH2, NO2, OH, Cl, and F, using NH3 as the common electron donor. The binding energy of XMeS···NH3 varies between 2.3 and 4.3 kcal/mol, with the strongest interaction occurring for X = F. The binding is strengthened by a factor of 2–10 in charged XH2S+···NH3 complexes, reaching a maximum of 37 kcal/mol for X = F. The binding is weakened to some degree when the H atoms are replaced by methyl groups in XMe2S+···NH3. The source of the interaction in the charged systems, like their neutral counterparts, is derived from a charge transfer from the N lone pair into the σ*(SX) antibonding orbital, supplemented by a strong electrostatic and smaller dispersion component. The binding is also derived from small contributions from a CH···N H-bond involving the methyl groups, which is most notable in the weaker complexes

Substituent Effects in the Noncovalent Bonding of SO2 to Molecules containing a Carbonyl Group. The Dominating Role of the Chalcogen Bond

The SO2 molecule is paired with a number of carbonyl-containing molecules, and the properties of the resulting complexes are calculated by high-level ab initio theory. The global minimum of each pair is held together primarily by a S···O chalcogen bond wherein the lone pairs of the carbonyl O transfer charge to the π* antibonding SO orbital, supplemented by smaller contributions from weak CH···O H-bonds. The binding energies vary between 4.2 and 8.6 kcal/mol, competitive with even some of the stronger noncovalent forces such as H-bonds and halogen bonds. The geometrical arrangement places the carbonyl O atom above the plane of the SO2 molecule, consistent with the disposition of the molecular electrostatic potentials of the two monomers. This S···O bond differs from the more commonly observed chalcogen bond in both geometry and origin. Substituents exert their influence via inductive effects that change the availability of the carbonyl O lone pairs as well as the intensity of the negative electrostatic potential surrounding this atom