3.2.3 Dipole Moment Surface of the van der Waals
Complex CH 4 –N 2
The description of dipole moments of molecular complexes is significantly
sophisticated as the number of atoms in the interacting molecules increases. The
electrical properties of molecular van der Waals complexes are now studied not
fully enough. Below, we concentrate our attention only on two relatively large
molecular complexes CH 4 –N 2 and C 2 H 2 –C 2 H 2 (Sect. 3.2.4) which have, on the one
hand, the astrophysical interest and on the other hand, they illustrate effectively the
theory discussed above.
It is well known that any gas media consisting of nonpolar molecules absorb in
IR and far IR spectra [32, 33]. The nature of absorption is in the presence of both
transient dipole moment of colliding molecules, and of the dipole moment of stable
van der Waals complexes. For this reason, the dipole moment of interacting
molecules is the object of numerous theoretical and experimental studies [24,
34–44]. One of the reasons arising an interest to the CH 4 –N 2 complex is related to
the study of the nitrogen-methane atmosphere of Titan [45–60] (the satellite of
Saturn). Due to low temperatures (70–100 °C [51]) of its atmosphere and relatively
high pressure (1.5 bar), the formation of stable van der Waals complexes
CH 4 –CH 4 , N 2 –N 2 and CH 4 –N 2 is very probable. These complexes play an
important role in all physical-chemical processes of Titan atmosphere [45, 46]. In
particular, van der Waals complex CH 4 –N 2 will manifest itself as spectral peculiarities on the background of non-resolved diffuse contours of absorption spectra of
colliding molecules CH 4 and N 2 [33].
At present, the CH 4 –N 2 complex is relatively well studied. There are several
works [61–63], where the theoretical investigation of the potential energy surface
was carried out. It was found that there is a family of the most stable configurations
of the CH 4 –N 2 complex with the energy difference of less then 0.04 cm
−1 [63].
Electric properties of the complex were studied in Refs. [7, 64]. In Ref. [7] the
analytical investigation of the long-range collision-induced dipole moment surface
taking into account the induction (up to R
−6 ) and dispersion (up to R
−7 ) contributions was carried out. The dipole moment surface suggested in Ref. [7], was
employed for the description of collisional spectra of molecules CH 4 and N 2 in
works [65, 66]. In these works, it was shown that calculated absorption spectra for
frequencies from 30 to 250 cm
−1 agrees well with existing measurements [67, 68]
but at the high frequencies >250 cm
−1 it shows substantial intensity defect. In the
work [64] (and Sect. 3.2.3) the long-range model of the dipole moment surface [64]
was improved by fully including the induction terms up to R
−7 and by accounting
for the contributions from the effects of electron shells overlap of the interacting
molecules.
A. Ab initio and analytical calculations
The calculation results of dipole moment surface of the van der Waals complex
CH 4 –N 2 are performed in this Section following to the theoretical treatment
30
3 Interaction-induced Dipole Moment
Complex CH 4 –N 2
The description of dipole moments of molecular complexes is significantly
sophisticated as the number of atoms in the interacting molecules increases. The
electrical properties of molecular van der Waals complexes are now studied not
fully enough. Below, we concentrate our attention only on two relatively large
molecular complexes CH 4 –N 2 and C 2 H 2 –C 2 H 2 (Sect. 3.2.4) which have, on the one
hand, the astrophysical interest and on the other hand, they illustrate effectively the
theory discussed above.
It is well known that any gas media consisting of nonpolar molecules absorb in
IR and far IR spectra [32, 33]. The nature of absorption is in the presence of both
transient dipole moment of colliding molecules, and of the dipole moment of stable
van der Waals complexes. For this reason, the dipole moment of interacting
molecules is the object of numerous theoretical and experimental studies [24,
34–44]. One of the reasons arising an interest to the CH 4 –N 2 complex is related to
the study of the nitrogen-methane atmosphere of Titan [45–60] (the satellite of
Saturn). Due to low temperatures (70–100 °C [51]) of its atmosphere and relatively
high pressure (1.5 bar), the formation of stable van der Waals complexes
CH 4 –CH 4 , N 2 –N 2 and CH 4 –N 2 is very probable. These complexes play an
important role in all physical-chemical processes of Titan atmosphere [45, 46]. In
particular, van der Waals complex CH 4 –N 2 will manifest itself as spectral peculiarities on the background of non-resolved diffuse contours of absorption spectra of
colliding molecules CH 4 and N 2 [33].
At present, the CH 4 –N 2 complex is relatively well studied. There are several
works [61–63], where the theoretical investigation of the potential energy surface
was carried out. It was found that there is a family of the most stable configurations
of the CH 4 –N 2 complex with the energy difference of less then 0.04 cm
−1 [63].
Electric properties of the complex were studied in Refs. [7, 64]. In Ref. [7] the
analytical investigation of the long-range collision-induced dipole moment surface
taking into account the induction (up to R
−6 ) and dispersion (up to R
−7 ) contributions was carried out. The dipole moment surface suggested in Ref. [7], was
employed for the description of collisional spectra of molecules CH 4 and N 2 in
works [65, 66]. In these works, it was shown that calculated absorption spectra for
frequencies from 30 to 250 cm
−1 agrees well with existing measurements [67, 68]
but at the high frequencies >250 cm
−1 it shows substantial intensity defect. In the
work [64] (and Sect. 3.2.3) the long-range model of the dipole moment surface [64]
was improved by fully including the induction terms up to R
−7 and by accounting
for the contributions from the effects of electron shells overlap of the interacting
molecules.
A. Ab initio and analytical calculations
The calculation results of dipole moment surface of the van der Waals complex
CH 4 –N 2 are performed in this Section following to the theoretical treatment
30
3 Interaction-induced Dipole Moment