104
P. R. P. Barreto et al.
energy surfaces of propylene oxide with helium [6, 7] and the study of the structural
isomers manifold of C 3 H 6 O [8]. Recent discovery in the interstellar medium of this
molecule [9] determined unusual interest, considering that numerous organic, even
complex, molecules are continuously discovered [10–12].
In this work, we present the study by a quantum mechanical approach of the binary
interaction between propylene oxide and the rare gas atoms He, Ne and Ar. Ab initio
calculations have been carried out at different levels of theory to determine the most
stable geometry of the molecule and the energy profiles of some significant configurations, called leading configurations (see also Refs. [13, 14]), chosen upon physical
and geometrical considerations. The ab initio points have been fitted with a fifthorder Rydberg potential function [15]. The Symmetry Adapted Perturbation Theory
(SAPT) method has been employed to calculate electric properties of the systems
propylene oxide–rare-gas atom, such as dipole moment, polarizability, quadrupole
moment, ionization potential, electron affinity and proton affinity. These properties
are employed for a preliminary analysis of electrostatic, exchange, induction and
dispersion contributions to the intermolecular forces.
The article is structured as follows: in Sect. 2, we give a background of the quantum
mechanical methods we employed and describe the leading configurations; in Sect. 3,
we discuss the results; in Sect. 4, final remarks end the paper.
2 Background
2.1 Geometry Optimization and Calculation of Potential
Energy Profile of the Leading Configurations
The structure of propylene oxide has been optimized by the Gaussian package [16] at
various levels of theory (Table 1). One hundred energy points have been calculated at
CCSD(T)/aug-cc-pVDZ by using MOLPRO [17] for a certain number of representative configurations, including the counterpoise correction to the basis set superposition error. As mentioned in Sect. 1, the choice of the representative configurations,
namely leading configurations, relay on geometric and physical considerations of the
system. The leading configurations of propylene oxide–He were discussed in Ref. [6]
(see also [18]) and the same choice is applied to the systems propylene oxide–Ne and
propylene oxide–Ar. Propylene oxide molecule is considered as built in a distorted
tetrahedron (Fig. 1), where the carbon atoms of CH 3 and CH 2 fragments, referred
as C3 and C2, the oxygen, O, and the hydrogen, H, atom bound to the asymmetric
carbon are the vertices. Fourteen configurations have been identified and indicated
by the letters V, E and F, that stand for vertex, edge and face of the tetrahedron,
respectively (Fig. 2). In the four V configurations, the rare-gas-atom lies through the
vertex–center-of-mass of propylene oxide direction. V1 is directed toward C3, V2 in
correspondence of C2, V3 toward O, and V4 along the H–center-of-mass direction
H. For the E configurations, the direction of the rare-gas-atom is referred to the line
P. R. P. Barreto et al.
energy surfaces of propylene oxide with helium [6, 7] and the study of the structural
isomers manifold of C 3 H 6 O [8]. Recent discovery in the interstellar medium of this
molecule [9] determined unusual interest, considering that numerous organic, even
complex, molecules are continuously discovered [10–12].
In this work, we present the study by a quantum mechanical approach of the binary
interaction between propylene oxide and the rare gas atoms He, Ne and Ar. Ab initio
calculations have been carried out at different levels of theory to determine the most
stable geometry of the molecule and the energy profiles of some significant configurations, called leading configurations (see also Refs. [13, 14]), chosen upon physical
and geometrical considerations. The ab initio points have been fitted with a fifthorder Rydberg potential function [15]. The Symmetry Adapted Perturbation Theory
(SAPT) method has been employed to calculate electric properties of the systems
propylene oxide–rare-gas atom, such as dipole moment, polarizability, quadrupole
moment, ionization potential, electron affinity and proton affinity. These properties
are employed for a preliminary analysis of electrostatic, exchange, induction and
dispersion contributions to the intermolecular forces.
The article is structured as follows: in Sect. 2, we give a background of the quantum
mechanical methods we employed and describe the leading configurations; in Sect. 3,
we discuss the results; in Sect. 4, final remarks end the paper.
2 Background
2.1 Geometry Optimization and Calculation of Potential
Energy Profile of the Leading Configurations
The structure of propylene oxide has been optimized by the Gaussian package [16] at
various levels of theory (Table 1). One hundred energy points have been calculated at
CCSD(T)/aug-cc-pVDZ by using MOLPRO [17] for a certain number of representative configurations, including the counterpoise correction to the basis set superposition error. As mentioned in Sect. 1, the choice of the representative configurations,
namely leading configurations, relay on geometric and physical considerations of the
system. The leading configurations of propylene oxide–He were discussed in Ref. [6]
(see also [18]) and the same choice is applied to the systems propylene oxide–Ne and
propylene oxide–Ar. Propylene oxide molecule is considered as built in a distorted
tetrahedron (Fig. 1), where the carbon atoms of CH 3 and CH 2 fragments, referred
as C3 and C2, the oxygen, O, and the hydrogen, H, atom bound to the asymmetric
carbon are the vertices. Fourteen configurations have been identified and indicated
by the letters V, E and F, that stand for vertex, edge and face of the tetrahedron,
respectively (Fig. 2). In the four V configurations, the rare-gas-atom lies through the
vertex–center-of-mass of propylene oxide direction. V1 is directed toward C3, V2 in
correspondence of C2, V3 toward O, and V4 along the H–center-of-mass direction
H. For the E configurations, the direction of the rare-gas-atom is referred to the line