Exploring Non-covalent Interactions by Jet-Cooled Electronic …
59
as solvent molecules. Thus, the intrinsic nature and optimum strength of the noncovalent interactions, as well as their very much existence on their own, should be
studied in an isolated condition, i.e., supersonic jet, in the absence of any solvent and
other external perturbation [102–106]. The experiments performed in a supersonic
jet go hand in hand with the quantum chemistry calculations. Hence, the performance
of various quantum chemical theories can also be verified directly by comparing the
theoretical results with the experimental ones.
In this chapter, we have discussed the investigation of the intrinsic properties
of various non-covalent interactions and their subtle interplay probed by isolated
gas phase laser spectroscopy. More specifically, jet-cooled electronic and vibrational
spectroscopy combined with quantum chemistry calculations performed on small
molecules and its complexes, mostly from the author’s laboratory, along with a brief
highlight from the other research groups in the world are described here.
2 Experimental Details
2.1 Supersonic Jet Expansion Technique
Supersonic jet-cooling technique is in the central part of the gas phase laser spectroscopy of isolated molecules and its weakly bound complexes held by various
non-covalent interactions [102–106]. An amalgamation of the supersonic jet technique with high-resolution tunable UV/Vis and IR lasers leads to well-resolved electronic and vibrational spectra of molecules and complexes. In general, solution-phase
UV-Vis electronic spectra of molecular systems measured at room temperature are
very broad. Hence, information on the vibronic structure of the molecules gets lost
underneath the broad background. The broad background in the solution phase electronic spectra emerges due to collisional broadening as well as a significant population of many rotational energy levels along with a small population of some of
the excited vibrational levels. On the other hand, mostly the lowest rotational (J" =
0) and vibrational (v" = 0) energy levels of the molecules in the ground electronic
state get populated in a supersonic jet, and the molecules remain in an isolated gas
phase without any collision with themselves as well as solvent [107, 108]. Consequently, very high-resolution electronic spectra featuring sharp vibronic bands of
the molecules are obtained. Additionally, the ultracold environment of the supersonic jet allows the formation of weakly bound complexes of different sizes and
measurements of their high resolution electronic and vibrational spectra, which are
not possible in the solution phase [103].
There are many excellent reviews in the literature describing the detailed principle and application of the supersonic jet-cooling technique in molecular spectroscopy [102–106, 109–114]. In supersonic jet-cooling technique, the vapor of
sample molecules of interest seeded in a carrier gas (He, Ne, Ar, etc.) of ~2–3 bar of
pressure is expanded into a high vacuum chamber (10
−6 –10
–7 mbar) through a small
59
as solvent molecules. Thus, the intrinsic nature and optimum strength of the noncovalent interactions, as well as their very much existence on their own, should be
studied in an isolated condition, i.e., supersonic jet, in the absence of any solvent and
other external perturbation [102–106]. The experiments performed in a supersonic
jet go hand in hand with the quantum chemistry calculations. Hence, the performance
of various quantum chemical theories can also be verified directly by comparing the
theoretical results with the experimental ones.
In this chapter, we have discussed the investigation of the intrinsic properties
of various non-covalent interactions and their subtle interplay probed by isolated
gas phase laser spectroscopy. More specifically, jet-cooled electronic and vibrational
spectroscopy combined with quantum chemistry calculations performed on small
molecules and its complexes, mostly from the author’s laboratory, along with a brief
highlight from the other research groups in the world are described here.
2 Experimental Details
2.1 Supersonic Jet Expansion Technique
Supersonic jet-cooling technique is in the central part of the gas phase laser spectroscopy of isolated molecules and its weakly bound complexes held by various
non-covalent interactions [102–106]. An amalgamation of the supersonic jet technique with high-resolution tunable UV/Vis and IR lasers leads to well-resolved electronic and vibrational spectra of molecules and complexes. In general, solution-phase
UV-Vis electronic spectra of molecular systems measured at room temperature are
very broad. Hence, information on the vibronic structure of the molecules gets lost
underneath the broad background. The broad background in the solution phase electronic spectra emerges due to collisional broadening as well as a significant population of many rotational energy levels along with a small population of some of
the excited vibrational levels. On the other hand, mostly the lowest rotational (J" =
0) and vibrational (v" = 0) energy levels of the molecules in the ground electronic
state get populated in a supersonic jet, and the molecules remain in an isolated gas
phase without any collision with themselves as well as solvent [107, 108]. Consequently, very high-resolution electronic spectra featuring sharp vibronic bands of
the molecules are obtained. Additionally, the ultracold environment of the supersonic jet allows the formation of weakly bound complexes of different sizes and
measurements of their high resolution electronic and vibrational spectra, which are
not possible in the solution phase [103].
There are many excellent reviews in the literature describing the detailed principle and application of the supersonic jet-cooling technique in molecular spectroscopy [102–106, 109–114]. In supersonic jet-cooling technique, the vapor of
sample molecules of interest seeded in a carrier gas (He, Ne, Ar, etc.) of ~2–3 bar of
pressure is expanded into a high vacuum chamber (10
−6 –10
–7 mbar) through a small