Intermolecular Potential Surface Research Articles

Ab initio potential energy surface and microwave spectrum of the NH3-N2 van der Waals complex.

We present a five-dimensional intermolecular potential energy surface (PES) of the NH3-N2 complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH3 symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH3 unit closest to the N2 unit, which is nearly parallel to the NH3 symmetry axis. The intermolecular distance is 7.01 a0, and the binding energy De is 250.6 cm-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH3 combines with (o/p)-N2, were calculated on this intermolecular potential surface. The computed dissociation energies D0 are 144.91 cm-1, 146.50 cm-1, 152.29 cm-1, and 154.64 cm-1 for (o)-NH3-(o)-N2, (o)-NH3-(p)-N2, (p)-NH3-(o)-N2, and (p)-NH3-(p)-N2, respectively. Guided by these calculations, the pure rotational transitions of the NH3-N2 van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole 14N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH3-N2 nuclear spin isomers.

Collision of cold CaF molecules: Towards evaporative cooling**Project supported by the National Natural Science Foundation of China (Grant Nos. 11604164 and U1810129), the Fund for Shanxi “1331 Project” Key Innovation Research Team, China (Grant No. 1331KIRT), and Excellent Youth Academic Leader in Higher Education of Shanxi Province, China (2018).

Long range intermolecular interaction potential surface of CaF (2Σ+) was simulated by employing the MOLPRO program and using the RCCSD(T)/def2-TZVP theory. The predicted data were further fitted to obtain the collision cross-section. The elastic collision cross-section of CaF at the temperature around 2 mK is as high as 6.5×10−9 cm2 and the collision rate is over 4.1×106 Hz. Additionally, we found that an orientation electric field will simplify the intermolecular interaction potential function from quaternary into ternary and the collision cross-section will be raised by about three orders. All-optical evaporative cooling of cold CaF is discussed in the conclusion.

The water-carbon monoxide dimer: new infrared spectra, ab initio rovibrational energy level calculations, and an interesting in-termolecular mode.

Bound state rovibrational energy level calculations using a high-level intermolecular potential surface are reported for H2O-CO and D2O-CO. They predict the ground K = 1 levels to lie about 20 (12) cm-1 above K = 0 for H2O-CO (D2O-CO) in good agreement with past experiments. But the first excited K = 1 levels are predicted to lie about 3 cm-1 below their K = 0 counterparts in both cases. Line strength calculations also indicate that mid-infrared transitions from the K = 0 ground state to this seemingly anomalous excited K = 1 state should be observable. These predictions are strikingly verified by new spectroscopic measurements covering the C-O stretch region around 2200 cm-1 for H2O-CO, D2O-CO, and HOD-CO, and the O-D stretch region around 2700 cm-1 for D2O-CO, HOD-CO, and DOH-CO. The experiments probe a pulsed supersonic slit jet expansion using tunable infrared quantum cascade laser or optical parametric oscillator sources. Discrete perturbations in the O-D stretch region give an experimental lower limit to the binding energy D0 of about 340 cm-1 for D2O-CO, as compared to our calculated value of 368 cm-1. Wavefunction plots are presented to help understand the intermolecular dynamics of H2O-CO. Coriolis interactions are invoked to explain the seemingly anomalous energies of the first excited K = 1 levels.

Ab initio potential and rotational spectra of the CO-N2 complex.

Ab initio calculations of the intermolecular potential energy surface (PES) of CO-N2 have been carried out using the closed-shell single- and double-excitation coupled cluster approach with a non-iterative perturbative treatment of triple excitations method and the augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set supplemented with midbond functions. The global minimum (De = 117.35 cm-1) of the four-dimensional PES corresponds to an approximately T-shaped structure with the N2 subunit forming the leg and CO the top. The bound rovibrational levels of the CO-N2 complex were calculated for total angular momenta J = 0-8 on this intermolecular potential surface. The calculated dissociation energies D0 are 75.60 and 76.79 cm-1 for the ortho-N2 (A-symmetry) and para-N2 (B-symmetry) nuclear spin modifications of CO-N2, respectively. Guided by these bound state calculations, a new millimeter-wave survey for the CO-N2 complex in the frequency range of 110-145 GHz was performed using the intracavity OROTRON jet spectrometer. Transitions not previously observed were detected and assigned to the subbands connecting the K = 0 and 1, (jCO, jN2 ) = (1, 0) states with a new K = 1, (jCO, jN2 ) = (2, 0) state. Finally, the measured rotational energy levels of the CO-N2 complex were compared to the theoretical bound state results, thus providing a critical test of the quality of the PES presented. The computed rovibrational wave functions were analyzed to characterize the nature of the different bound states observed for the two nuclear spin species of CO-N2.

Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.

Rotational study of the NH3-CO complex: millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex NH3-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112-139 GHz. Newly observed and assigned transitions belong to the K = 0-0, K = 1-1, K = 1-0, and K = 2-1 subbands correlating with the rotationless (jk)NH3 = 00 ground state of free ortho-NH3 and the K = 0-1 and K = 2-1 subbands correlating with the (jk)NH3 = 11 ground state of free para-NH3. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH3-CO and para-NH3-CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH3-CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy De = 359.21 cm(-1). The bound rovibrational levels of the NH3-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 210.43 and 218.66 cm(-1) for ortho-NH3-CO and para-NH3-CO, respectively.

Influence of velocity effects on the shape of N2 (and air) broadened H2O lines revisited with classical molecular dynamics simulations

The modeling of the shape of H(2)O lines perturbed by N(2) (and air) using the Keilson-Storer (KS) kernel for collision-induced velocity changes is revisited with classical molecular dynamics simulations (CMDS). The latter have been performed for a large number of molecules starting from intermolecular-potential surfaces. Contrary to the assumption made in a previous study [H. Tran, D. Bermejo, J.-L. Domenech, P. Joubert, R. R. Gamache, and J.-M. Hartmann, J. Quant. Spectrosc. Radiat. Transf. 108, 126 (2007)], the results of these CMDS show that the velocity-orientation and -modulus changes statistically occur at the same time scale. This validates the use of a single memory parameter in the Keilson-Storer kernel to describe both the velocity-orientation and -modulus changes. The CMDS results also show that velocity- and rotational state-changing collisions are statistically partially correlated. A partially correlated speed-dependent Keilson-Storer model has thus been used to describe the line-shape. For this, the velocity changes KS kernel parameters have been directly determined from CMDS, while the speed-dependent broadening and shifting coefficients have been calculated with a semi-classical approach. Comparisons between calculated spectra and measurements of several lines of H(2)O broadened by N(2) (and air) in the ν(3) and 2ν(1) + ν(2) + ν(3) bands for a wide range of pressure show very satisfactory agreement. The evolution of non-Voigt effects from Doppler to collisional regimes is also presented and discussed.

Rovibrational states of the H2O–H2 complex: An ab initio calculation

All bound rovibrational levels of the H(2)O-H(2) dimer are calculated for total angular momentum J = 0-5 on two recent intermolecular potential surfaces reported by Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] and Hodges et al. [J. Chem. Phys. 120, 710 (2004)] obtained through ab initio calculations. The method used handles correctly the large amplitude internal motions in this complex; it involves a discrete variable representation of the intermolecular distance coordinate R and a basis of coupled free rotor wave functions for the hindered internal rotations and the overall rotation of the dimer. The basis is adapted to the permutation symmetry associated with the para/ortho (p/o) nature of both H(2)O and H(2) as well as to inversion symmetry. Dimers containing oH(2) are more strongly bound than dimers with pH(2), as expected, with dissociation energies D(0) of 33.57, 36.63, 53.60, and 59.04 cm(-1)for pH(2)O-pH(2), oH(2)O-pH(2), pH(2)O-oH(2), and oH(2)O-oH(2), respectively, on the potential of Valiron et al. that corresponds to a binding energy D(e) of 235.14 cm(-1). Rovibrational wave functions are computed as well and the nature of the bound states in the four different dimer species is discussed. Converged rovibrational levels on both potentials agree well with the high-resolution spectrum reported by Weida and Nesbitt [J. Chem. Phys. 110, 156 (1999)]; the hindered internal rotor model that was used to interpret this spectrum is qualitatively correct.

Accurate correlated calculation of the intermolecular potential surface in the coronene dimer

The binding energy of the coronene dimer was determined by high level ab initio calculations. In the first step a potential energy surface was determined for the stacked dimer at the SCS-MP2/aTZ level. The energy of the stacked parallel dimer was scanned with respect to the lateral displacement of the two molecules at various interplane distances, and the minima and saddle points were determined. These results were refined by high level quadratic CI (QCISD(T)/aDZ) calculations. The aTZ and aDZ basis sets employed are derived from the aug-cc-pVDZ and aug-cc-pVTZ basis sets but diffuse functions are placed only at every second carbon atom, alleviating the severe numerical difficulties of the full augmented basis sets while retaining all important contributions. To estimate the QCISD(T) energies for larger basis set, we employed commonly used procedure of adding basis set correction values obtained at the MP2 level. This hybrid scheme was used to obtain the final optimised geometries and binding energies; these are believed to be close approximations to the basis set limit QCISD(T) values. A few calculations were also performed to the T-shaped conformer, although this is less important in the coronene dimer than in the benzene dimer. The high-level results were used as a benchmark to assess the performance of simpler and less expensive ab initio methods for these large rings with small HOMO–LUMO gaps, for instance the SCS-MP2 method, and modern DFT versions that are optimised for dispersion dominated systems.

Vibration–rotation-tunneling states of the benzene dimer: an ab initio study

An improved intermolecular potential surface for the benzene dimer is constructed from interaction energies computed by symmetry-adapted perturbation theory, SAPT(DFT), with the inclusion of third-order contributions. Twelve characteristic points on the surface have been investigated also using the coupled-cluster method with single, double, and perturbative triple excitations, CCSD(T), and triple-zeta quality basis sets with midbond functions. The SAPT and CCSD(T) results are in close agreement and provide the best representation of these points to date. The potential was used in calculations of vibration-rotation-tunneling (VRT) levels of the dimer by a method appropriate for large amplitude intermolecular motions and tunneling between multiple equivalent minima in the potential. The resulting VRT levels were analyzed with the use of the permutation-inversion full cluster tunneling (FCT) group G(576) and a chain of subgroups that starts from the molecular symmetry group C(s)(M) of the rigid dimer at its equilibrium C(s) geometry and leads to G(576) if all possible intermolecular tunneling mechanisms are feasible. Further information was extracted from the calculated wave functions. It was found, in agreement with the experimental data, that for all of the 54 G(576) symmetry species (with different nuclear spin statistical weights) the lower VRT states have a tilted T-shape (TT) structure; states with the parallel-displaced structure are higher in energy than the ground state of A symmetry by at least 30 cm(-1). The dissociation energy D(0) equals 870 cm(-1), while the depth D(e) of the TT minimum in the potential is 975 cm(-1). Hindered rotation of the cap in the TT structure and tilt tunneling lead to level splittings on the order of 1 cm(-1). Also intermolecular vibrations with excitation energies starting at a few cm(-1) were identified. A further small, but probably significant, level splitting was assigned to cap turnover, although in scans of the potential surface we could not find a plausible 'reaction path' for this process. Rotational constants were extracted from energy levels calculated for total angular momentum J = 0 and 1, and from expectation values of the inertia tensor. Although the end-over-end rotational constant B + C agrees well with the measured microwave spectra, there is disagreement with the measurements concerning the (a)symmetric rotor character of the benzene dimer. It is concluded from calculations for the 54 nuclear spin species that the microwave spectrum should show overlapping contributions from many different species. Another interesting conclusion regards the role of the quantum number K, for a prolate near-symmetric rotor the projection of the total angular momentum on the prolate axis. For the benzene dimer, K has a substantial effect on the energy levels associated with the intermolecular motions of the complex.

Theoretical study of noncovalent interactions between triple bonds and chlorine atoms in complexes of acetylene and some chloromethanes

Ab initio calculations were performed to investigate the noncovalent interactions between the triple bonds and chlorine atoms in the complexes of acetylene with several chloromethanes, CH 3Cl, CH 2Cl 2, CHCl 3 and CCl 4. The intermolecular potential surfaces of the acetylene–chloromethane complexes containing C–Cl⋯π (triple bond) interaction were calculated by the HF and MP2 methods with Dunning’s correlation consistent basis sets (cc-pVXZ, X = D, T and Q). At the minimum, the intermolecular distances between the triple bond and chlorine atom range from 3.3 to 3.5 Å. The CCSD(T) method predicted that the binding energies of the acetylene complexes with CH 3Cl, CH 2Cl 2, CHCl 3 and CCl 4 are −0.49, −0.77, −1.05 and −1.34 kcal/mol at the basis set limit, respectively. The energy decomposition analysis indicates that the complex is mainly stabilized by the dispersion interaction while the electrostatic interaction also plays an important role in the attraction between acetylene and chloromethane molecules.

Lone pair ··· π interactions between water oxygens and aromatic residues: Quantum chemical studies based on high‐resolution protein structures and model compounds

The pi electron cloud of aromatic centers is known to be involved in several noncovalent interactions such as C--H...pi, O--H...pi, and pi...pi interactions in biomolecules. Lone-pair (lp) ... pi interactions have gained attention recently and their role in biomolecular structures is being recognized. In this article, we have carried out systematic analysis of high-resolution protein structures and identified more than 400 examples in which water oxygen atoms are in close contact (distance < 3.5 A) with the aromatic centers of aromatic residues. Three different methods were used to build hydrogen atoms and we used a consensus approach to find out potential candidates for lp...pi interactions between water oxygen and aromatic residues. Quantum mechanical calculations at MP2/6-311++G(d,p) level on model systems based on protein structures indicate that majority of the identified examples have energetically favorable interactions. The influence of water hydrogen atoms was investigated by sampling water orientations as a function of two parameters: distance from the aromatic center and the angle between the aromatic plane and the plane formed by the three water atoms. Intermolecular potential surfaces were constructed using six model compounds representing the four aromatic amino acids and 510 different water orientations for each model compound. Ab initio molecular orbital calculations at MP2/6-311++G(d,p) level show that the interaction energy is favorable even when hydrogen atoms are farthest from the aromatic plane while water oxygen is pointing toward the aromatic center. The strength of such interaction depends upon the distance of water hydrogen atoms from the aromatic substituents. Our calculations clearly show that the lp...pi interactions due to the close approach of water oxygen and aromatic center are influenced by the positions of water hydrogen atoms and the aromatic substituents.

Some Formal Aspects of the Theory of Intermolecular Interactions and of the BSSE Problem

Some results of mathematical character concerning the theory of intermolecular interactions and the BSSE problem are presented. It is shown that the concept of complete basis set may be introduced for intermolecular potential surfaces only by considering explicitly the limiting process in which the basis sets of both monomers approach completeness simultaneously. That does not lead to any overcompleteness problem if we do not postulate the existence of two complete basis sets from the outset. The intimate connection between the BSSE and the differences of some biorthogonal integrals and their "original" counterparts is also discussed. The operator of BSSE is given in terms of such differences. It is shown that in a special case, when only the overlap of the occupied orbitals is considered, the "bi-expectation" value of the energy coincides with the conventional expectation value for the single determinant wave function built up of the unperturbed orbitals of the individual monomers. It is discussed, by using a model of the biorthogonal perturbation theory, why the conceptually fully different a priori (CHA) and a posteriori (CP) schemes of BSSE correction usually give very close numerical results. (The necessary biorthogonal perturbation formalism is developed in the Appendices.) The results give justification for the additivity assumptions inherent in the CP method.

An empirical potential-energy surface for the He–I2(BΠu3) van der Waals complex including three-body effects

An empirical intermolecular potential surface is proposed for the He-I2(B 3IIu) complex, modeled as a sum of pairwise He-I Morse interactions plus a three-body interaction term. The potential reproduces with very good agreement the spectral blueshifts and vibrational predissociation lifetimes measured for He-I2(B,v') in the range v' = 10-67 of I2 vibrational excitations. In particular, the accuracy achieved in the description of the experimental data for high v' levels is attributed to the three-body interaction term included in the potential. The behavior of the potential surface with the I-I separation is analyzed and correlated with the experimental findings.

Structure and energy difference of two isomers of He–CH3F

The intermolecular potential surface of He-CH(3)F is investigated through ab initio calculations and microwave and millimeter-wave spectroscopies. The intermolecular potential is calculated at the fourth-order Møller-Plesset level with a large basis set including bond functions. Three minimums exist, the deepest of which is at the carbon end of the C-F axis and has a depth of 46.903 cm(-1), the second deepest is in a T-shaped position relative to the C-F axis with a depth of 44.790 cm(-1), and the shallowest is at the fluorine end of the C-F axis with a depth of 30.929 cm(-1). The barrier to internal rotation of the CH(3)F subunit about its C-F axis is very low, thus leading to essentially free internal rotation and two separate sets of bound states correlating to ortho-CH(3)F (|K| = 3n) for the ground, or A, internal rotor state upon which this study focuses, and to para-CH(3)F (|K| = 3n +/- 1) for the excited, or E, internal rotor state. Bound-state calculations of the A state performed using two different techniques show the lowest-energy state to have the helium localized in the T-shaped well with an energy of -11.460 cm(-1), while two excited configurations of the A state have the helium localized either in the well at the carbon end ("linear") with an energy of -7.468 cm(-1) or in the well at the fluorine end ("antilinear") with an energy of -4.805 cm(-1). Spectroscopic observations confirm the predicted energy-level structure of the ground and first excited states. Sixteen transitions between 12 distinct energy levels have been observed, including pure rotational transitions of both the T-shaped ground state and the linear excited state, as well as rovibrational transitions between the ground state and the linear excited state. The energy difference between the T-shaped state and the linear state is measured to be 132 374.081(16) MHz. There is significant Coriolis mixing of the ground state J(K(a)K(c)) = 2(20) and the linear J(K) = 2(0) levels which aided in the observation of the T to linear transitions. This mixing and the T to linear energy difference are sensitive probes of the relative well depths of the two lowest minimums and are well predicted by the ab initio potential. Improved agreement between experiment and theory is obtained by morphing the correlation energy of the potential. He-CH(3)F is one of just a few atom-molecule complexes for which the ground-state geometry does not coincide with the global potential minimum.

Ab initio study for the intermolecular interaction potential surface of Ar-N2 complex

The intermolecular interaction potentials of van der Waals Ar-N-2 complex have been studied by ab initio calculations using the single and double excitation coupled cluster [CCSD(T)] theory with perturbative triples correction. The full counterpoise method is applied to correct the basis set superposition error (BSSE). It is found that the T-shaped structure is the most stable conformation with the well depth D-e of 12.40 meV at the minimum distance R-m of 3.70 A. The calculated anisotropic values for U R-m, U R-0 and U D-e are 0.56 A, 0.54 A and 2.68 meV, respectively. Compared with those obtained by others, our calculated PES seems to be in better agreement with experiments.

Structure and bonding of ArClF: Intermolecular potential surface

AbstractThe ground‐state potential surface of the Ar‐ClF complex is determined by restricted coupled‐cluster with single and double excitations and a noniterative perturbation treatment of triple excitation (RCCSD(T)) calculations, using augmented correlation consistent basis sets supplemented with an additional set of midbond functions. Three local minima are found for the Ar‐ClF, which correspond to linear Ar‐Cl‐F and Ar‐F‐Cl configurations and a near T‐shaped structure. The well depths and the equilibrium distances are 283.62 cm−1 at 3.84 Å and 128.96 cm−1 at 4.31 Å for the linear ones and 173.13 cm−1 at 3.58 Å for the one at θ = 107.5°.Bound‐state calculations are carried out and the linear Ar‐Cl‐F structure is found to be the most stable one, in accordance with the experiment. Dissociation energies, intermolecular distances, and vibrational frequencies are computed and compared with available experimental data.

Testing the Mason–Monchick approximation for the calculation of the viscosity of carbon dioxide

The Mason–Monchick approximation (MMA) was used to calculate the viscosity cross-section, S (2 0 0 0), for carbon dioxide using the most recent intermolecular potential surfaces. The MMA values were compared with the results of full classical trajectory calculations. The deviations obtained were small, at worst 3.4% in magnitude and decreasing with increasing temperature. This rather surprising agreement was traced to the cancellation of MMA errors in estimating two terms that enter the viscosity cross-section. Thus, the MMA may not be reliable as a general tool for testing the accuracy of proposed intermolecular surfaces using viscosity data.

The van der Waals potential energy surfaces and structures of He–ICl and Ne–ICl clusters

The potential energy surfaces of the ground electronic state of rare gas interhalogen van der Waals molecules, Rg–ICl (Rg=He, Ne), are calculated at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Calculations have been performed with specific augmented correlation consistent basis sets for the noble atom (Rg), supplemented with an additional set of bond functions. For iodine atom a correlation consistent triple zeta valence basis set in conjunction with large-core Stuttgart–Dresden–Bonn relativistic pseudopotential has been employed. The CCSD(T) results predict the existence of three minima on the Rg–ICl potential energy surfaces at collinear (Rg–ICl), antilinear (Rg–ClI), and near T-shaped configurations, with the collinear structure to be the lowest one. Bound states calculated from the intermolecular potential surfaces show that zero-order vibrational corrections do not alter the stability of the three structures. Equilibrium intermolecular distances, binding energies, and isomerization barriers are evaluated using the CCSD(T) potentials and compared with previous theoretical and/or experimental results.

The Slippery Sliding Interaction of Acetylene with Polyynes

Ab initio calculations were carried out for dimers of acetylene and a polyyne chain with two to five triple bonds at the MP2 level. The structure and energies of several symmetry-constrained structures were found and used to guide refinement of a model potential that could be applied to longer polyyne chains. The slice of the intermolecular potential surface for sliding an acetylene along the polyyne chain in a T-shaped orientation was mapped and showed shallow corrugation over a large region. This amounts to slipperiness in this type of weak bonding.