Exploring Non-covalent Interactions by Jet-Cooled Electronic …
79
115, 145, 173, 196]. However, it is reported in the literature that the IR red-shift
in the stretching frequency of the hydrogen-bonded functional group arises from
the electrostatic, polarization, and charge transfer interactions while the dispersion
interaction contributes to the overall stability of the system [80].
Absolutely localized molecular orbital energy decomposition analysis (ALMOEDA) [197, 198] of different conformers of indole…dimethylselenide (indmse1,
indmse2), indole…dimethylsulfide (indms1, indms2) and indole…dimethyloxide
(indmo) presented in Fig. 10B illustrates that there is a linear relationship between the
IR red-shift of the N–H stretching frequency and charge-transfer interaction present
in the complexes. It should be pointed out here that indmse2 and indms2 were not
observed in the experiment, although those conformers are stable low energy minima.
The reason for not observing these two conformers could be due to their low interconversion barrier with the global minima. In the ALMO-EDA method, total interaction
energy (E) is decomposed into frozen density (E Frz ), polarization (E Pol ), and
charge transfer (CT) components, where E Frz consists of electrostatic, repulsion,
and dispersion energy. This result demonstrates that the CT interaction in combination with the electrostatic and polarization interactions has a significant contribution
to the observed IR red-shift in the stretching frequency of the hydrogen bond donor
in the case of the unconventional hydrogen bonds [145].
Das and co-workers further modeled single water-mediated selenium hydrogen
bonding interactions present in proteins by studying 1:1:1 trimeric complex of indole,
dimethyl selenide, and water using isolated gas-phase UV-IR double resonance spectroscopy [193]. Here, indole and dimethyl selenide represent the side chains of the
tryptophan and selenomethionine amino acid residues of proteins, respectively. It has
been found from detailed PDB analysis that there were 7526 single water-mediated
Se hydrogen bonding interactions present in proteins. Figure 11Bc shows a massselected conformation-specific IR spectrum of indole…dimethylselenide…H 2 O
measured in the N-H and O-H stretching frequency region using RIDIR spectroscopy. A comparison of the experimental IR spectrum of the trimer with the
theoretical IR spectrum (Fig. 11Bd) reveals that the observed trimer has a cyclic
structure where H 2 O makes a bridge between indole and dimethyl selenide through
N-H…O and O-H…Se double hydrogen-bonding interactions. It could be noted
that the observed structure is stabilized by both conventional (N–H…O) and unconventional (O–H…Se) hydrogen bonding interactions. The hydrogen-bonded N-H
and O-H stretching frequencies in the trimer appear at 3399 and 3414 cm
−1 , respectively. Interestingly, the most stable structure of the indole…dimethylselenide…H 2 O
complex observed in the experiment corroborate the structural motifs of the single
water-mediated Se hydrogen bonding interactions found between the amino acid
residues in proteins and Fig. 11A shows a representative analogy of this [193].
79
115, 145, 173, 196]. However, it is reported in the literature that the IR red-shift
in the stretching frequency of the hydrogen-bonded functional group arises from
the electrostatic, polarization, and charge transfer interactions while the dispersion
interaction contributes to the overall stability of the system [80].
Absolutely localized molecular orbital energy decomposition analysis (ALMOEDA) [197, 198] of different conformers of indole…dimethylselenide (indmse1,
indmse2), indole…dimethylsulfide (indms1, indms2) and indole…dimethyloxide
(indmo) presented in Fig. 10B illustrates that there is a linear relationship between the
IR red-shift of the N–H stretching frequency and charge-transfer interaction present
in the complexes. It should be pointed out here that indmse2 and indms2 were not
observed in the experiment, although those conformers are stable low energy minima.
The reason for not observing these two conformers could be due to their low interconversion barrier with the global minima. In the ALMO-EDA method, total interaction
energy (E) is decomposed into frozen density (E Frz ), polarization (E Pol ), and
charge transfer (CT) components, where E Frz consists of electrostatic, repulsion,
and dispersion energy. This result demonstrates that the CT interaction in combination with the electrostatic and polarization interactions has a significant contribution
to the observed IR red-shift in the stretching frequency of the hydrogen bond donor
in the case of the unconventional hydrogen bonds [145].
Das and co-workers further modeled single water-mediated selenium hydrogen
bonding interactions present in proteins by studying 1:1:1 trimeric complex of indole,
dimethyl selenide, and water using isolated gas-phase UV-IR double resonance spectroscopy [193]. Here, indole and dimethyl selenide represent the side chains of the
tryptophan and selenomethionine amino acid residues of proteins, respectively. It has
been found from detailed PDB analysis that there were 7526 single water-mediated
Se hydrogen bonding interactions present in proteins. Figure 11Bc shows a massselected conformation-specific IR spectrum of indole…dimethylselenide…H 2 O
measured in the N-H and O-H stretching frequency region using RIDIR spectroscopy. A comparison of the experimental IR spectrum of the trimer with the
theoretical IR spectrum (Fig. 11Bd) reveals that the observed trimer has a cyclic
structure where H 2 O makes a bridge between indole and dimethyl selenide through
N-H…O and O-H…Se double hydrogen-bonding interactions. It could be noted
that the observed structure is stabilized by both conventional (N–H…O) and unconventional (O–H…Se) hydrogen bonding interactions. The hydrogen-bonded N-H
and O-H stretching frequencies in the trimer appear at 3399 and 3414 cm
−1 , respectively. Interestingly, the most stable structure of the indole…dimethylselenide…H 2 O
complex observed in the experiment corroborate the structural motifs of the single
water-mediated Se hydrogen bonding interactions found between the amino acid
residues in proteins and Fig. 11A shows a representative analogy of this [193].