Microwave observation of the "recently found" polar OCS dimer

Recently Afshari et al. reported on the detection of a new infrared band which was assigned to the "long-anticipated polar isomer of the OCS dimer" [J. Chem. Phys. 126, 071102 (2007)]. The authors report here the microwave confirmation of their results. The lowest energy, nonpolar isomer of (OCS)2 has long been known from IR spectroscopy, while the polar form has only been deduced from qualitative beam refocusing experiments. The higher energy, polar isomer of (OCS)2 has been produced by high pressure expansion of dilute OCS in helium. A surprisingly strong microwave spectrum of Cs (OCS)2 has been observed and assigned.     

New infrared bands of nonpolar OCS dimer and experimental frequencies for two intermolecular modes

Spectra of the nonpolar carbonyl sulfide dimer in the region of the OCS ν(1) fundamental band were observed in a slit-jet supersonic expansion. The jet was probed using radiation from a tunable diode laser employed in a rapid-scan signal averaging mode. Six new bands were observed and analyzed, all of which originate from the dimer ground vibrational state. Three were vibrational fundamentals involving the ((18)OCS)(2) and (16)OCS-(18)OCS isotopologues. They enabled an estimate to be made of the frequency of the infrared-forbidden mode corresponding to in-phase vibration of the OCS monomers in the dimer, a value needed to obtain an intermolecular vibrational frequency from one of the observed combination bands. A relatively weak b-type dimer band centered at 2103.105 cm(-1) was assigned to the OCS 4ν(2) (l = 0) bending overtone. Combination bands were observed involving the geared bend and van der Waals stretch intermolecular modes. The resulting experimental frequencies of 37.5(20) cm(-1) for the bend and 42.9727(1) cm(-1) for the stretch are in good agreement with a recent high level theoretical calculation.     

Computational study of the rovibrational spectrum of (OCS)2

In this paper, we report a new intermolecular potential energy surface and rovibrational transition frequencies and line strengths computed for the OCS dimer. The potential is made by fitting energies obtained from explicitly correlated coupled-cluster calculations and fit using an interpolating moving least squares method. The rovibrational Schroedinger equation is solved with a symmetry-adapted Lanczos algorithm and an uncoupled product basis set. All four intermolecular coordinates are included in the calculation. On the potential energy surface we find, previously unknown, cross-shaped isomers and also polar and non-polar isomers. The associated wavefunctions and energy levels are presented. To identify polar and cross states we use both calculations of line strengths and vibrational parent analysis. Calculated rotational constants differ from their experimental counterparts by less than 0.001 cm(-1).     

Infrared spectra of the C2H2-(OCS)2 van der Waals complex: observation of a structure with C2 symmetry

Infrared spectra of the C(2)H(2)-(OCS)(2) trimer are studied by means of direct infrared absorption spectroscopy. The van der Waals complexes are generated in a supersonic slit-jet apparatus and probed using a rapid-scan tunable diode laser in the region of the ν(1) fundamental vibration of the OCS monomer. Two infrared bands are analyzed for the lowest energy isomer of the trimer, which has C(2) symmetry and is experimentally observed here for the first time. A relatively strong band centered at 2068.93 cm(-1) is assigned as the out-of-phase vibrations of the pair of equivalent OCS monomers. This band is blue-shifted relative to the free OCS monomer but with a reduced shift as compared with the analogous vibration of the nonpolar OCS dimer. A weaker red-shifted band observed at 2049.64 cm(-1) establishes the nonplanarity of the OCS dimer subunit within the trimer. Spectra for three isotopologues in addition to the normal form are used to help define an experimental structure, which agrees well with past and present semiempirical calculations.     

Far infrared absorption in OCS and CS2 up to the critical region

The far infrared (15–200 cm⁻¹) spectra of OCS and CS₂ up to the critical point are reported. The experimental integrated intensities are calculated and, for OCS, compared to the theoretical one. One is led to the conclusion that induced-dipole absorption exists in the polar OCS as well as in non-polar CS₂.     

Nitrous oxide dimer: observation of a new polar isomer

Spectra of the nitrous oxide dimer (N2O)2 are studied in the region of the N2O nu1 fundamental band around 2230 cm-1 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. The previously known band of the centrosymmetric nonpolar dimer is analyzed in improved detail, and a new band is observed and assigned to a polar isomer of (N2O)2. This polar form of the dimer has a slipped parallel structure, rather similar to the slipped antiparallel structure of the nonpolar form but with a slightly larger intermolecular distance. The accurate rotational parameters determined here should enable a microwave observation of the polar N2O dimer. The need for a modern ab initio investigation of the N2O-N2O intermolecular potential energy surface is emphasized.     

Qualitative structure of (CO2)2 and (OCS)2

Molecular beam deflection studies on (CO₂)₂ and (OCS)₂ indicate that both these species are polar molecules. Structural implications of this are explored in light of previous studies of these systems.     

Ab initio potential energy surface and predicted microwave spectra for Ar--OCS dimer and structures of Arn--OCS (n = 2-14) clusters

An ab initio potential energy surface for the Ar--OCS dimer was calculated using the coupled-cluster singles and doubles with noniterative inclusion of connected triples [CCSD(T)] with a large basis set containing bond functions. The interaction energies were obtained by the supermolecular approach with the full counterpoise correction for the basis set superposition error. The CCSD(T) potential was found to have two minima corresponding to the T-shaped and the collinear Ar--SCO structures. The two-dimensional discrete variable representation method was employed to calculate the rovibrational energy levels for five isotopomers Ar--OCS, Ar--OC34S, Ar--O13CS, Ar--18OCS, and Ar--17OCS. The calculated pure rotational transition frequencies for the vibrational ground state of the five isotopomers are in good agreement with the observed values. The corresponding microwave spectra show that the b-type transitions (Delta Ka = +/-1) are significantly stronger than the a-type transitions (Delta Ka = 0). Minimum-energy structures of the Ar2--OCS trimer were been determined with MP2 optimization, whereas the minimum-energy structures of the Arn--OCS clusters with n = 3-14 were obtained with the pairwise additive potentials. It was found that there are two minima corresponding to one distorted tetrahedral structure and one planar structure for the ternary complex. The 14 nearest neighbor Ar atoms form the first solvation shell around the OCS molecule.     

"Knock-out" analogues as a tool to quantify supramolecular processes: a theoretical study of molecular interactions in guanidiniocarbonyl pyrrole carboxylate dimers

It was recently shown experimentally that 5-(guanidiniocarbonyl)-1H-pyrrole-2-carboxylate 1, a self-complementary zwitterion, dimerizes even in water with an unprecedented high association constant of K = 170 M(-1) (J. Am. Chem. Soc. 2003, 125, 452-459). To get an insight into the importance of the various noncovalent binding interactions and of their interplay (electrostatic interactions, hydrogen binding, cooperative effects), we employ density functional theory to study the stability of several "knock-out" analogues in which single hydrogen bonds within these multiple point binding motif are switched off by replacing N-H hydrogen-donor groups with either methylene groups or an oxygen ether bridge. The influence of a highly polar solvent on the dimer stabilities is also examined. These calculations reproduce the experimental data for zwitterion 1. A comparison of 1 with the arginine dimer shows that the energy contents of the monomers also significantly influence the dimer stabilities. The analysis of the various "knock-out" analogues reveals as a main conclusion that simple models either based just on hydrogen-bond counting or on the assumption that the charge interaction by itself is the main and dominant factor fail to explain the stability of such self-assembled dimers. Our computations show that the hydrogen-bond network, the electrostatic attraction, and also their mutual interactions are responsible for the high stability of zwitterion 1.     

Molecular recognition in methanol: the first example of hydrogen-bond-mediated self-association of a calix[4]arene in polar,protic solvent

Tetraalaninecalixarene was prepared by coupling of tetraaminocalix[4]arene with alanine. It dimerizes in methanol, providing the first example of a substituted calixarene that undergoes self-association through hydrogen bonding in polar, protic solvent. The association constant in 24:1 MeOH:H2O was determined to be 29 000 M-1. Addition of arginine or lysine results in disruption of the dimer and formation of a 1:1 complex between the amino acid and the tetraalaninecalixarene. The preparation of a peptidocalixarene that associates in polar solvent opens new doors for the use of calixarenes for molecular recognition in biologically relevant environments.     

Spectroscopic observation and structure of CS2 dimer

Infrared spectra of the CS(2) dimer are observed in the region of the CS(2) ν(3) fundamental band (～1535 cm(-1)) using a tunable diode laser spectrometer. The weakly bound complex is formed in a pulsed supersonic slit-jet expansion of a dilute gas mixture of carbon disulfide in helium. Contrary to the planar slipped-parallel geometry previously observed for (CO(2))(2), (N(2)O)(2), and (OCS)(2), the CS(2) dimer exhibits a cross-shaped structure with D(2d) symmetry. Two bands were observed and analyzed: the fundamental (C-S asymmetric stretch) and a combination involving this mode plus an intermolecular vibration. In both cases, the rotational structure corresponds to a perpendicular (ΔK = ±1) band of a symmetric rotor molecule. The intermolecular center of mass separation (C-C distance) is determined to be 3.539(7) A?. Thanks to symmetry, this is the only parameter required to characterize the structure, if the monomer geometry is assumed to remain unchanged in the dimer. From the band centers of the fundamental and combination band an intermolecular frequency of 10.96 cm(-1) is obtained, which we assign as the torsional bending mode. This constitutes the first high resolution spectroscopic investigation of CS(2) dimer.     

Thermochemical stabilities and structures of the cluster ions OCS+, S2+, H+(OCS), and C2H5+ with OCS molecules in the gas phase

The gas-phase clustering reactions of OCS+, S2+, H+(OCS), and C2H5+ ions with carbonyl sulfide (OCS) molecules were studied using a pulsed electron-beam high-pressure mass spectrometer and applying density functional theory (DFT) calculations. In the cluster ions OCS+(OCS)(n) and H+(OCS)(OCS)(n), a moderately strong, here referred to as "semi-covalent", bond was formed with n = 1. However, the nature of bonding changed from semi-covalent to electrostatic with n = 1 --> 2. The bond energy of S2(+)(OCS) was determined experimentally to be 12.9 +/- 1 kcal/mol, which is significantly smaller than that of the isovalent S2(+)(CS2) complex (30.9 +/- 1.5 kcal/mol). DFT based calculations predicted the presence of several isomeric structures for H+(OCS)(OCS)(n) complexes. The bond energies in the C2H5+(OCS)(n) clusters showed an irregular decrease for n = 1 --> 2 and 7 --> 8. The nonclassical bridge structure for the free C2H5+ isomerized to form a semi-covalent bond with one OCS ligand, [H3CCH2...SCO]+, i.e., reverted to classical structure. However, the nonclassical bridge structure of C2H5+ was preserved in the cluster ions C2H5+(OCS)(n) below 140 K attributable to the lack of thermal energy for the isomerization. DFT calculations revealed that stability orders of the geometric isomers of H+(OCS)(OCS)(n) and C2H5+(OCS)(n) changed with increasing n values.     

Polar isomer of formic acid dimers formed in helium nanodroplets

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the "free" OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O-H...O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole-dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O-H...O and one C-H...O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.     

Columnar mesomorphism of bent-rod mesogens containing 1,2,4-oxadiazole rings

In this study, the synthesis, characterization, and mesomorphic properties of ten new bent-rod compounds containing two units of 1,2,4-oxadiazoles are reported. In order to understand the relationship between the structure and the mesomorphic behavior, molecules containing a variety of polar substituents (i.e., I, NO₂, NH₂, OH) on the central rigid core were prepared. The hexagonal columnar mesomorphism was characterized by DSC and POM and the nature of the mesophases was established through XRD studies. The driving force for self-assembly can be explained by microsegregation between the aliphatic parts and the polar parts, producing a dimer, trimer, and tetramer inside a single disc.     

Dimerization of formic acid--an example of a "noncovalent" reaction mechanism

The pulse deposition technique allows selectively the isolation of monomeric or dimeric formic acid in argon matrices at 7 K. Warming of matrices containing the monomer M from 7 K to 40 K results in the decrease of M and formation of a dimer B. This dimer is also labile, and further warming finally produces a second dimer A. By comparison with density functional theory (DFT) calculations and gas phase IR spectra taken from the literature, the latter dimer A was identified as the C2h-symmetrical cyclic dimer. The unstable dimer B was identified as the acyclic Cs-symmetrical dimer. An activation energy of 2.3 kcal mol(-1) was calculated for the B --> A rearrangement at the B3LYP/ 6-311 ++ G(d,p) level of theory, which is in qualitative agreement with the experimental finding of a slow thermal reaction under the conditions of matrix isolation.     

Photoinduced Charge Transfer in a Conformational Switching Chlorin Dimer–Azafulleroid in Polar and Nonpolar Media

In the present study, a biomimetic reaction center model, that is, a molecular triad consisting of a chlorin dimer and an azafulleroid, is synthesized and its photophysical properties are studied in comparison with the corresponding molecular dyad, which consists only of a chlorin monomer and an azafulleroid. As evidenced by ¹H NMR, UV/Vis, and fluorescence spectroscopy, the chlorin dimer–azafulleroid folds in nonpolar media into a C₂‐symmetric geometry through hydrogen bonding, resulting in appreciable electronic interactions between the chlorins, whereas in polar media the two chlorins diverge from contact. Femtosecond transient absorption spectroscopy studies reveal longer charge‐separated states for the chlorin dimer–azafulleroid; ≈1.6 ns in toluene, compared with the lifetime of ≈0.9 ns for the corresponding chlorin monomer–azafulleroid in toluene. In polar media, for example, benzonitrile, similar charge‐separated states are observed, but the lifetimes are inevitably shorter: 65 and 73 ps for the dimeric and monomeric chlorin–azafulleroids, respectively. Nanosecond transient absorption and singlet oxygen phosphorescence studies corroborate that in toluene, the charge‐separated state decays indirectly via the triplet excited state to the ground state, whereas in benzonitrile, direct recombination to the ground state is observed. Complementary DFT studies suggest two energy‐minima conformations, that is, a folded chlorin dimer–azafulleroid, which is present in nonpolar media, and another conformation in polar media, in which the two hydrophobic chlorins wrap the azafulleroid. Inspection of the frontier molecular orbitals shows that in the folded conformation, the HOMO on each chlorin is equivalent and is shared owing to partial π–π overlap, resulting in delocalization of the conjugated π electrons, whereas the wrapped conformation lacks this stabilization. As such, the longer charge‐separated lifetime for the dimer is rationalized by both the electron donor–acceptor separation distance and the stabilization of the radical cation through delocalization. The chlorin folding seems to change the photophysical properties in a manner similar to that observed in the chlorophyll dimer in natural photosynthetic reaction centers.     

Super-molecule calculations of intermolecular forces between long-chain molecules

Intermolecular interactions between paraffin chains (C₃C₉) in four different mutual orientations have been studied by PCILO and INDO methods. The PCILO method is found to be superior. It is observed that polarity optimization is not required for these interaction studies although it most certainly would be for molecules with more polar bonds. The calculated interaction energies per carbon atom extrapolated to long-chain length are (0.09–0.22) kcal mol^?1 depending upon the orientation and geometry. The effect of a polar head group on the intermolecular interactions of long-chain molecules is also studied using the propionic acid dimer as model. Lone-pair interactions cause one of the dimer orientations to be totally repulsive.     

Infrared spectroscopic investigation of two isomers of the weakly bound complex OCS-(CO2)2

Vibration-rotation spectra of the OCS-(CO(2))(2) van der Waals complex were studied by means of direct infrared absorption spectroscopy. Complexes were generated in a supersonic slit-jet apparatus, and the expansion gas was probed using a rapid-scan tunable diode laser. Infrared bands were observed for two different isomeric forms of the complex. A relatively strong band centered at 2058.799 cm(-1) was assigned to the most stable isomer, which has a barrel-shaped geometry and is already known from microwave spectroscopy. A weaker infrared band centered at 2050.702 cm(-1) was assigned to a new isomeric form, observed here for the first time, which was expected on the basis of ab initio calculations. Infrared bands for seven isotopomers were recorded in an attempt to quantify the structure of the new isomer. Because it has no symmetry elements, nine parameters are needed to fully define the geometry. It was possible to determine six of these which define the relative position of the OCS monomer with respect to the CO(2) dimer fragment in the complex while the remaining three were fixed at their ab initio values. Similarities and differences between the faces of the two isomers of OCS-(CO(2))(2) and the associated dimers are discussed.     

Is combining meta‐GGA correlation functionals with the OPTX exchange functional useful?

The recent generalized gradient approximation (GGA) density functional OCS1 of Handy and Cohen is implemented in the deMon code and tested on a carefully selected set of problems. OCS1 is found to be accurate for molecular atomization energies, transition metal–ligand bonds, and systems with intramolecular hydrogen bonds. However, OCS1 encounters problems for systems with intermolecular hydrogen bonds. It also tends to elongate bond lengths systematically, and sometimes significantly. The OPTX exchange is combined with three meta‐GGA correlation functionals, Lap3, τ1, and τ2, the latter reported for the first time. The new meta‐GGA scheme OPTX exchange plus τ2 correlation called Oτ2 yields improved molecular geometries, NMR shielding constants, and an improved barrier height for the H+H₂ reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Fluorescent Short-Stranded Helical Foldamers Based on L-shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine

Helical structures were constructed by using π-spacer-bridged dimers of dibenzopyrrolo[1,2-a][1,8]naphthyridine, which has a highly fluorescent L-shaped π-extended skeleton. Three dimers with biphenylene (dimer 1 ), phenanthrene (dimer 2 ), and m-phenylene (dimer 3 ) spacers, as well as a fixed-helical dimer 4 where two quinolone rings were covalently cross-linked, were designed and prepared. ¹H NMR and ROESY spectra revealed that dimers 1 and 2 adopted helical forms in solution, whereas dimer 3 did not. The helical conformation of 1 was strengthened by addition of either polar or nonpolar solvents to the chloroform solution, which suggested that π–π stacking was the main contributor to the stabilization of the helical structure. All of the dimers, including fixed-helical dimer 4 , emitted fluorescence with high quantum yields (ϕ=0.79–0.86).