Rotational Spectroscopic Investigations Of CH 4 ---H 2 S Complex Aiswarya Lakshmi P. and E. Arunan Inorganic and Physical Chemistry Indian Institute of Science Bangalore, INDIA OSU International Symposium on Molecular Spectroscopy June 21-25, 2010

“The hydrogen bond can be defined as an attractive interaction between two molecular moieties at least one of them contains hydrogen atom that plays a fundamental role in the interaction” Alkorta*, 1. Rozas and J. Elguero, Ber. Bunsenges. Phys. Chem. 102, (1998). Electron rich region can act as a hydrogen bond acceptor Electron rich region can act as a hydrogen bond acceptor Lone pair electrons, pi-bond, unpaired electron, sigma bond and Lone pair electrons, pi-bond, unpaired electron, sigma bond and hydrides act as H-bond acceptors hydrides act as H-bond acceptors Conventional hydrogen bond acceptors are F, N, O, P, S, Se, Cl, I Conventional hydrogen bond acceptors are F, N, O, P, S, Se, Cl, I Introduction

Hydrocarbons?? “The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule or fragment in which there is evidence of bond formation.” Recommendation submitted by the IUPAC task group ( )

C 6 H 6 … H 2 O C 6 H 6 … HCN C 6 H 6 … H 2 S C 2 H 4 … H 2 S C 2 H 4 … H 2 O

CH 4 ??? Spherical top molecule Spherical top molecule Lowest non-vanishing multipole moment  Octupole Moment Lowest non-vanishing multipole moment  Octupole Moment CH 4 forms hydrogen bonded complex with HX (X=F, Cl, OH, SH) Raghavendra et al. Chem. Phys. Lett. 467 (2008)37 Mausumi et al. Phys. Chem. Chem. Phys. 11 (2009) 8974 Zero point energy along the torsional coordinate that can break the H-bond, should be below the barrier along the coordinate

ZPE (Case1) ZPE (Case2)  If the zero point energy is greater than the torsional energy, internal rotation is possible  Main objective of the study is to obtain the equilibrium structure and to obtain the torsional barrier

Hydrogen bond donors approach along this region to form the ‘hydrogen bond’. Electron Density Plot Centre of each tetrahedron plane Region of high electron density Where is the electron rich region in CH 4 ?

 Microwave spectroscopy has confirmed the structure of CH 4 HF / HCl / H 2 O  CH 4 acts as hydrogen bond acceptor CH 4 H 2 S ???? ComplexΔE BSSE (kJ mol -1 ) H 4 CHF-8.8 H 4 CHCl-9.6 H 4 CH 2 O-6.3 H 4 CH 2 S-6.7 D. Suenram, G. T. Fraser, F. J. Lovas, Y. Kawashima, J. Chem. Phys. 101(9) 1994, 7230 A. C. Legon, B. P. Roberts, A. L. Wallwork, Chem. Phys. Letters, 173(1) 1990, 107

Various Possible structures: Ab initio Results:

 E (kcal/mol) -1.6 r(C-H) (Å) Å  CHS177.6   E(kcal/mol) -1.2 r(S-H) (Å) Å  CHS178.2  H 4 CH 2 S CH 4SH 2 CH 4 as hydrogen bond acceptor is more stable Optimized Structures:

Potential Energy surfaces: Rotation of CH 4 molecule “Floppy structure”  Energy (hartree)

H 2 S (I = 0,1) CH 4 (I = 0,1,2) Depending on the coupling, a large number of states will be present C H 4 H 2 O 14 states were observed 6 states  internal angular momentum, m =1

Pulsed Nozzle Fourier transform Microwave Spectrometer Range  2 to 26 GHz and Resolution  1kHz

Pulsed Nozzle Fourier Transform Microwave Spectrometer  Pulsed Nozzle Fourier Transform Microwave Spectrometer  rotational spectrum of the CH 4 ---H 2 S complex and its isotopomers rotational spectrum of the CH 4 ---H 2 S complex and its isotopomers CH 4 -HF and CH 4 -H 2 O are similar, B = MHz and CH 4 -HF and CH 4 -H 2 O are similar, B = MHz and MHz respectively MHz respectively CH 4 - H 2 S may have similar behavior as CH 4 -HCl CH 4 - H 2 S may have similar behavior as CH 4 -HCl (J=0  MHz) (J=0  MHz) The first transition was observed at MHz The first transition was observed at MHz Search has been done for ~ 2 GHz and 2 J=0  1 transitions observed Search has been done for ~ 2 GHz and 2 J=0  1 transitions observed Three progressions were observed in J=1  2 region Three progressions were observed in J=1  2 region

J=0  1 J=1  2 J=2  Three progressions were observed Three progressions were observed Fitted independently to a linear top Fitted independently to a linear top One progression  Internal angular momentum, m=1 One progression  Internal angular momentum, m=1 8 Transitions were observed CH 4 -H 2 S Transitions:

Ground State rotational constant  (1) MHz Ground State rotational constant  (1) MHz Distortion Constant, D J  (9) MHz Distortion Constant, D J  (9) MHz Intermolecular separation  4.136Å Intermolecular separation  4.136Å J=0  J=1  J=2  Progression: I

J=0  J=1  J=2  Rotational constant  (1) MHz Rotational constant  (1) MHz Distortion Constant, D J  (7) MHz Distortion Constant, D J  (7) MHz Negative distortion constant  rotational - vibrational coupling Negative distortion constant  rotational - vibrational coupling The progression arises from some excited internal rotor/torsional state The progression arises from some excited internal rotor/torsional state Progression: II

CH 4 -D 2 S Transitions: J=1  2J=2  J=1  2J=2  CH 4 -HDS Transitions:

ComplexB (MHz)D J (MHz) CH 4 -H 2 S (1) (9) CH 4 -D 2 S CH 4 -HDS Calculated: CH 4 -H 2 S CH 4 -SH Ch 4 -D 2 S CH 4 -SD CH 4 -HDS CH 4 -SHD It cannot be concluded whether CH 4 is interacting with hydrogen or sulphur

Rotational spectrum confirms the formation of CH H 2 S complex Rotational spectrum confirms the formation of CH H 2 S complex More experimental data is required to ascertain whether the type of interaction is More experimental data is required to ascertain whether the type of interaction is “CH HSH or CH SH 2 ”

Results from Atoms In Molecules calculations Koch and Popelier have proposed eight criteria U. Koch, P. L. A. Popelier J. Phys. Chem, 1995, 99, Topology: Presence of bond critical point and bond path HBCP HBCP

Penetration parameters r°Ar°A rArA  rA r°Hr°H rHrH  rH H 4 C H 2 S CH 4 SH H 2 O HCl Mutual Penetration of Hydrogen and Acceptor Atom  L H 4 CH 2 S CH 4 SH H 2 O HCl  (BCP)  [ ] au L  [-0.15, 0.02] au , electron density, L(r), laplacian of electron density at bcp

Change in the population of H bonded hydrogen MonomerComplexDifference H 4 CH 2 S CH 4 SH H 2 O HCl Loss of charge of hydrogen atom Destabilized hydrogen atom Change in atomic energies MonomerComplexDifference H4C H2S CH4 SH H2O HCl

Change in atomic first moments MonomerComplexDifference H4C H2S CH4 SH H2O HCl Change in H volume MonomerComplexDifference H 4 C H 2 S CH 4 SH H 2 O HCl Decrease of dipolar polarization of hydrogen atom Decrease in the volume of hydrogen atom

Electron densities at the BCPs are within the range suggested for hydrogen bond Electron densities at the BCPs are within the range suggested for hydrogen bond AIM parameters are in agreement with the value for a bond to be H-bond AIM parameters are in agreement with the value for a bond to be H-bond CH 4  better H-bond acceptor than donor CH 4  better H-bond acceptor than donor

Conclusion: Rotational spectrum confirms the formation of CH 4 ---H 2 S complex Rotational spectrum confirms the formation of CH 4 ---H 2 S complex More experimental data are required to ascertain whether it is CH 4 -HSH or CH 4 -SH 2 type of interaction More experimental data are required to ascertain whether it is CH 4 -HSH or CH 4 -SH 2 type of interaction Ab initio calculations and AIM results show that CH 4 -HSH interaction is more stable Ab initio calculations and AIM results show that CH 4 -HSH interaction is more stable All the AIM parameters are in agreement with the value for a bond to be H-bond All the AIM parameters are in agreement with the value for a bond to be H-bond “However, the rotational spectrum indicates that the complex is very floppy. Zero point energy level should be above the barrier for internal rotation. Hence, there is unlikely to be an orientational preference, found in hydrogen bond.”

Acknowledgement: Department of Inorganic and Physical Chemistry Indian Institute of Science Indo –French Centre for Promotion of Advanced Research