Rotational Spectra and Structure of Phenylacetylene-Water Complex and Phenylacetylene-H 2 S (preliminary) Mausumi Goswami, L. Narasimhan, S. T. Manju and E. Arunan Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore TA 04, 63 rd International Symposium on Molecular Spectroscopy, Columbus, 2008

Legon-Millen’ s rules Reference: Legon and Millen, Chem. Soc. Rev., 16, 1987, 467 Geometry of B---HX, hydrogen bonded complexes Rule 1 says “if ‘B’ has lone-pair, the axis of H-X molecule coincides with that of lone- pair” Rule 2 says “if ‘B’ has no non-bonding electron pair but has  -bonding electron pairs, the axis of the H-X molecule intersects the internuclear axis of the atoms forming the  - bond and is perpendicular to the plane of symmetry of the  -bond” Rule 3: “Rule 1 is definitive when ‘B’ has both non-bonding and  -bonding pairs.” These rules were derived from a large set of experimental data where X=F, Cl, CN and ‘B’ is having either lone-pair or  -electrons or both.

Phenyl ring  -cloud: could be hydrogen-bond acceptor Acetylenic  -cloud: could be hydrogen-bond acceptor Acetylenic C-H: could be hydrogen-bond donor Where will H 2 O bind? Would interaction with H 2 S be similar or different?

Fluorobenzene-HCl 1 :ClH---  -bonded structure Fluorobenzene-H 2 O 2,3 : nearly planar with C-H---O and O-H---F Benzonitrile-H 2 O has a structure similar to the above. Strength of the hydrogen bond donors affects the global minimum! 1. Lopez, Kuczkowski and co-workers, J. Chem. Phys., 118, 2003, Jäger and co-workers, Columbus meeting, P. Tarakeshwar, Kwang S. Kim and B. Brutschy, J. Chem. Phys., 110, 1999, From earlier work

Phenylacetylene-water: ab initio (G. N. Patwari and co-workers) Binding energy =11.2 kJmol -1 A=2083MHZ, B=1132 MHz, C=992 MHz Binding energy = 11.3 kJmol -1 A=2678 MHz, B=998.0 MHz, C=729 MHz Binding energy = 8.7 kJmol -1 A=5518MHz, B=506MHz, C=464MHz A B C G. N. Patwari and co-workers, J. Phys. Chem. A, 112, 2008, 3360

IR-UV double resonance studies (G. N. Patwari and co-workers ) Phenylacetylene monomer: Fermi resonance between acetylenic C-H stretch and a combination of one quantum of C  C stretch and two quanta of C  C-H bend. 1 Phenylacetylene-water acetylenic C-H stretch region disappearance of the Fermi resonance in the acetylenic C-H stretch region (shift of -3 cm -1. Rules out the structure A) Phenylacetylene-water O-H stretch region: 3724 cm -1 (free O-H stretch) 3629cm -1 (H-bonded O-H stretch) G. N. Patwari and co-workers, J. Phys. Chem. A, 112, 2008, 3360

What can rotational spectroscopy tell us new? Unambiguous structural information. Insight into large amplitude motions that lead to splitting of rotational transitions (unresolvable with IR-UV double resonance). Hyperfine splitting and dipole moments.

Pulsed Nozzle Fourier Transform Microwave Spectrometer Arunan, Tiwari, Mandal and Mathias Curr. Sci. 2002

Initial search in Argon from GHz One weak signal depending on phenylacetylene and water could be seen after averaging for gas pulses in Argon. The signal appears in 100 shots in Helium. Argon-phenylacetylene lines Argon-phenylacetylene (Dreizler and co-workers, J. Mol. Str., 825, 2006, 1) Some transitions were newly measured, but could be fit with the same Hamiltonian as reported by Dreizler

4(0,4)-5(0,5) Search using Helium shows a doubling of the transitions

TransitionsSeries1Res. (kHz)Series2Res. (kHz) Lines in red were not found in the initial searches with Ar. Search in He led to the observation of these lines

Fitted parameters for Phenylacetylene-H 2 O ParametersSeries1 (Stronger) Series2 (Weaker) A (MHz) (3) (3) B (MHz) (8) (9) C (MHz) (4) (4) d 1 (kHz)0.077(3)0.091(4) d 2 (kHz)1.1(1)1.5(1) DJDJ 0.366(6)0.392(8) D JK -0.58(2)-0.69(2) DkDk 5.1(6)6.9(5) Sd(kHz) #1# Number of fitted transitions

TransitionsFrequenciesRes. (kHz) Phenylacetylene-HOD Only one series of transitions found!

Fitted parameters for Phenylacetylene-HOD Parameters A (MHz) (4) B (MHz) (1) C (MHz) (7) d 1 (kHz)0.092(5) d 2 (kHz)1.5(2) D J (kHz)0.38(1) D JK (kHz)-0.7(1) D k (kHz)5.4(8) Sd (kHz)3.9 #1# Number of fitted transitions Search and assignment for D 2 O isotopomer is on progress. A few lines have been identified. Some lines show splittings, yet to ascertain whether the splitting is due to hyperfine or tunneling.

Structure of Phenylacetylene-H 2 O A (MHz)2672 B (MHz)996 C (MHz)731 B Theory A (MHz)2678 B (MHz)998.0 C (MHz)729 Experiment What is the splitting due to?

Possible cause of splitting B Interchange of the two hydrogens. Free internal rotation as observed in C 6 H 6 -H 2 O/Ar-H 2 O is unlikely. Flipping of non-bonded hydrogen

Nature of interactions What are the different interactions present in this complex? We use Atoms in Molecules theory to address this question.

Atoms in Molecules Analysis OH  interaction C-HO interaction Two ring critical points Electron density at BCP for O-H---pi is a.u at BCP for C-H---O is a.u

Ab initio prediction: Phenylacetylene-H 2 S Calculation at MP2(full)/ G** and MP2/aug-cc-pVDZ True minima located with binding energy=3.5 kcalsmol -1 A (MHz)1269 B (MHz)1176 C (MHz)782 Saddle point of order 5!! A (MHz)2162 B (MHz)644 C (MHz) Å 3.10 Å

Search for a minima having C-H---S is on A5396 MHz B329 MHz C311 MHz Expected rotational constants Few promising lines appearing near the predictions for the  - bonded structure have been observed. Search and assignments are on progress.

Some promising lines observed during search for phenylacetylene-H 2 S complex ~20 more transitions have been observed. Assignments are in progress. For the S-H  complex, transitions were predicted with about 1.4 GHz spacing and three such transitions have been found

Conclusions Microwave spectrum of phenylacetelyne-H 2 O unambiguously confirms the nearly planar structure involving both O-H  and C-HO interactions. Each transition is observed as a doublet for H 2 O and a singlet for HDO. Atoms in molecules theory predicts both interactions to be equally strong. Phenylacetylene-H 2 S, theory and preliminary rotational spectrum suggest that the structure has only S-H  interaction with the H 2 S perpendicular to the plane of phenylacetylene. Legon-Millen rules need to be refined for molecules with multiple possibilities.

Acknowledgements Prof. Naresh Patwari, IIT, Bombay Department of Science and Technology, India Indo-French centre for Promotion of Advanced Research, India.