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Rotational Spectroscopic Investigations Of CH4---H2S Complex

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					      Rotational Spectroscopic
         Investigations Of
        CH4---H2S 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
       Introduction

    “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”


Electron rich region can act as a hydrogen bond acceptor

Lone pair electrons, pi-bond, unpaired electron, sigma bond and
 hydrides act as H-bond acceptors

Conventional hydrogen bond acceptors are F, N, O, P, S, Se, Cl, I


      Alkorta*, 1. Rozas and J. Elguero, Ber. Bunsenges. Phys. Chem. 102, 429-435 (1998).
Recommendation submitted by the IUPAC task group
               (2004-026-2-100)



 “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.”



         Hydrocarbons??
C6H6 … H2O        C6H6 … HCN          C6H6 … H2S




     C2H4 … H2S                C2H4… H2O
        CH4 ???
 Spherical top molecule

 Lowest non-vanishing multipole moment  Octupole Moment


            CH4 forms hydrogen bonded complex with HX
                             (X=F , Cl , OH , SH)



   Zero point energy along the torsional coordinate that can break
    the H-bond , should be below the barrier along the coordinate


                 Raghavendra et al. Chem. Phys. Lett. 467 (2008)37
              Mausumi et al. Phys. Chem. Chem. Phys. 11 (2009) 8974
                                                              ZPE (Case2)




                                                          ZPE (Case1)




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
Where is the electron rich region in CH4?

                 Electron Density Plot


                Centre of each tetrahedron plane



                 Region of high electron density

               Hydrogen bond donors approach
                 along this region to form the
                      ‘hydrogen bond’.
       Microwave spectroscopy has confirmed the structure of
                  CH4 ••• HF / HCl / H2O

                       CH4 acts as hydrogen bond acceptor



                         CH4 ••• H2S ????
                     Complex                  ΔEBSSE (kJ mol-1)
                   H4C•••HF                           -8.8
                   H4C•••HCl                          -9.6
                  H4C•••H2O                           -6.3
                  H4C•••H2S                           -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
             Ab initio Results:
Various Possible structures:
      Optimized Structures:

             H4C•••H2S                   CH4•••SH2




E (kcal/mol)      -1.6         E(kcal/mol)    -1.2

r(C-H) (Å)         2.6029 Å     r(S-H) (Å)      3.0246 Å

CHS               177.6       CHS            178.2 


         CH4 as hydrogen bond acceptor is more stable
            Potential Energy surfaces:
 Rotation of CH4 molecule




                        Energy (hartree)
                                           -439.32




                                           -439.33




                                           -439.34



“Floppy structure”                         -439.35




                                           -439.36


                                                     0   50   100       150   200   250

                                                                    
                H2S (I = 0,1)
                CH4 (I = 0,1,2)

Depending on the coupling,
                a large number of states will be present


                       C H4 ••• H2O
                    14 states were observed
          6 states  internal angular momentum, m =1
Pulsed Nozzle Fourier transform Microwave
              Spectrometer
                                                    1000mm
                            4'' GATE VALVE                        CAJON COUPLING

                                                                              4'' TO 8'' PORT ADAPTOR FLANGE
                  PENNING
                   GAUGE
                                                                           PIRANI GAUGE
          LINEAR BEARING                                                                  ANTENNA


                                                            FIXED                          HINGED DOOR




                                                   mm
                                                   500
                                                           MIRROR
GASES                                                                                          MICROWAVE SOURCE
                                                         750 mm
  SCREW ROD FINE PITCH
                                                   MOVABLE MIRROR
                               20 mm
                                                                                               GUIDE ROD

            STEPPER
             MOTOR
                      PULSED                                                         LIQUID N TRAP
                                                                                             2
                      NOZZLE
                                                                                PIRANI GAUGE

                        STAND

                  20'' DIFFUSION                                                     BOOSTER PUMP
                       PUMP

        WATER CIRCULATION PIPES



                                                                                      ROTARY                   EXHAUST
                                                                                   VACUUM PUMP


                                                                                                     80
        TO COOLING TOWER




                                       WATER FLOW SWITCH




        Range  2 to 26 GHz and Resolution  1kHz
 Pulsed Nozzle Fourier Transform Microwave Spectrometer 
 rotational spectrum of the CH4---H2S complex and its isotopomers

 CH4-HF and CH4-H2O are similar, B = 4238.5328 MHz and
 4346.7202MHz respectively

 CH4- H2S may have similar behavior as CH4-HCl
 (J=01 5815.8202 MHz)

 The first transition was observed at 5365.827 MHz

 Search has been done for ~ 2 GHz and 2 J=01 transitions observed


 Three progressions were observed in J=12 region
            CH4-H2S Transitions:

        8 Transitions were observed

         J=01       J=12           J=23
         5186.105    10372.485       15559.236
                     10583.4049      15870.1485
         5365.827    10729.387       16088.437



 Three progressions were observed
 Fitted independently to a linear top
 One progression  Internal angular momentum, m=1
                   Progression: I

              J=01              5365.827
              J=12              10729.387
              J=23              16088.437



 Ground State rotational constant  2683.100(1) MHz
 Distortion Constant, DJ  0.09413(9) MHz
 Intermolecular separation  4.136Å
                     Progression: II
                  J=01                5186.105

                  J=12                10372.485
                  J=23                15559.236


        Rotational constant  2593.05(1) MHz
        Distortion Constant, DJ  -0.0089(7) MHz

 Negative distortion constant  rotational - vibrational coupling
 The progression arises from some excited internal rotor/torsional state
CH4-D2S Transitions:

    J=12        J=23
   10498.663    15745.0489
  10157.9088




CH4-HDS Transitions:

    J=12         J=23
   10631.1755   15941.1215

   10278.2462
 Complex       B (MHz)        DJ (MHz)

 CH4-H2S       2683.100(1)    0.09413(9)

 CH4-D2S       2625.0584      0.04909

 CH4-HDS       2658.5461      0.09402


Calculated:
CH4-H2S       2680.4357      CH4-SH2       2742.1061
Ch4-D2S       2645.3432      CH4-SD2       2626.5521
CH4-HDS       2678.7711      CH4-SHD       2688.3127


    It cannot be concluded whether CH4 is
     interacting with hydrogen or sulphur
Rotational spectrum confirms the formation of
 CH4 --- H2S complex


More experimental data is required to ascertain
 whether the type of interaction is

        “CH4 --- HSH or CH4 --- SH2”
Results from Atoms In Molecules calculations

        Koch and Popelier have proposed eight criteria

Topology: Presence of bond critical point and bond
          path



                  HBCP                        HBCP




   U. Koch, P. L. A. Popelier J. Phys. Chem, 1995, 99, 97472-9754
     , electron density, L(r), laplacian of electron
                                 density at bcp

                                    L
  H4C•••H2S           0.0074         -0.0072
                                                     (BCP)  [0.002- 0.04] au
  CH4 ••• SH2         0.0062         -0.0043
                                                     L  [-0.15, 0.02] au
  H2O ••• HCl         0.0350         -0.0239


  Mutual Penetration of Hydrogen and Acceptor Atom

Penetration parameters
                r°A        rA         rA      r°H       rH           rH
H4C ••• H2S 2.0            1.6       0.4       1.4       1.0         0.4
CH4 ••• SH2     2.3        1.9       0.4       1.4       1.1         0.3
H2O ••• HCl 1.9            1.2       0.7       1.3       0.6         0.7
   Loss of charge of hydrogen atom
Change in the population of H bonded hydrogen
                Monomer         Complex         Difference
H4C•••H2S       1.0846          1.0911          0.0065
CH4 ••• SH2     0.9984          0.9928          -0.0056
H2O ••• HCl     0.7591          0.6718          -0.0873


   Destabilized hydrogen atom

Change in atomic energies
                Monomer         Complex         Difference
H4C ••• H2S     -0.6573         -0.6555         0.0018

CH4 ••• SH2     -0.6366         -0.6298         0.0068

H2O ••• HCl     -0.5312         -0.4824         0.0488
    Decrease of dipolar polarization of hydrogen atom
Change in atomic first moments

                Monomer          Complex   Difference

H4C ••• H2S     0.0275           0.0103    -0.0172
CH4 ••• SH2     0.1413           0.1269    -0.0144
H2O ••• HCl     0.1234           0.0651    -0.0583


 Decrease in the volume of hydrogen atom

Change in H volume
                Monomer          Complex   Difference
H4C ••• H2S     55.4723          53.3986   -2.0737
CH4 ••• SH2     52.0764          51.4332   -0.6432
H2O ••• HCl     40.0266          26.1095   -13.9171
 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


 CH4  better H-bond acceptor than donor
                               Conclusion:

Rotational spectrum confirms the formation of CH4 ---H2S complex
More experimental data are required to ascertain whether it is CH 4-HSH or CH4-SH2
type of interaction
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



    “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

				
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