Pradeep R. Varadwaj*, Arpita Varadwaj, Gilles H. Peslherbe A Computational Investigation of c-C 3 H 2  HX (X = F, Cl, Br) H-bonded Complexes Department of Chemistry & Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montréal, Québec, Canada, H4B 1R6 * 66 th OHIO STATE UNIVERSITY INTERNATIONAL SYMPOSIUM ON MOLECULAR SPECTROSCOPY June 20-24, 2011 FB03

Hydrogen Bonding: A Versatile Interaction Contributing factors: To stabilize a hydrogen bond  Electrostatic  Short-range exchange  Polarization  Induction  Charge transfer  Dispersion (London type) A-X  H-Y N  H-O, N  H-N, O  H-O, O  H-N, C  H-O, C  H-N, O  H-C, N  H-C X, Y = electronegative atoms

Possible Potential Energy Surfaces of H-bond assisted systems Asymmetric double-well potential Symmetric double-well potential Low-barrier H-bond potential Single-well potential O…H-O H-atom is attached more strongly to O (covalently bonded) High barrier to H-atom transfer C. L. Perrin et al, Annu. Rev. Phys. Chem. 48 (1997) C. L. Perrin, Science 266 (1994) 1665 – 1668; ACC. Chem. Res. 43 (2010) H-atom is delocalized across a wide region Proton resides at the center

Interest & Motivation Fragments Cyclic – C 3 H 2 HX (X = F, Cl, Br) c-C 3 H 2 is a very important carbene of significant astrophysical interest, and it has found significant application in synthetic chemistry as well in transition metal catalysis and organic catalysis c-C 3 H 2  HX (X = F, Cl, Br) We are interested in examining  The efficiency of c-C 3 H 2 (i) as a proton donor, and (ii) as a proton acceptor  The singlet state (it is the most stable form which is feasible experimentally)  The potential energy surface (relaxed and rigid) to find the global minimum  The effect of an inappropriate basis set and of correlated functionals in the study of H-bonding  The amount of proton transfer (if any) and related signatures in identifying the existence of H-bonding in these complexes.

c-C 3 H 2  HF complex 0.88 Input to Gaussian Å Å Output from Gaussian B3LYP/aug-cc-pVTZ results  = Isolated molecules  = Å  = 1.71 D  = Free Complex 874 cm km mol -1 Vibrational Properties

To locate the global minimum of c-C 3 H 2  HF complex in conjunction with four different basis sets, cc-pVDZ, cc-pVTZ, and their augmented analogues aug-cc- pVDZ and aug-cc-PVTZ. The plots are drawn between the scan ordinate  C– H  F (in degrees) and total electronic energy (in kcal mol -1 ). MP2 level relaxed potential energy surface scan 2.1 kcal

 MP2/aug-cc-pVQZ level equilibrium configurations of c-C 3 H 2  HX complexes  i), ii), and iii) represent complexes containing HF, HCl and HBr, respectively.  (a) the charge transfer (  Q/e) from the carbene lone pair electrons to the anti-bonding orbital  *(HX) and, the direction of  Q is shown by an arrow in blue. Each upper and lower line entries in (b) and (c) represents the value of charge (or  Q) obtained from Bader- and natural population analysis schemes, respectively.  (b) The Bader and NPA based atomic charges given for selected atoms (involved in H-bond formation) as in the complexes – An indication of charge rearrangement on H-bond formation.  (c) the inter-molecular and halogen halide bond distances (in Å ).

Complex typeBasis  EbEb E b zpe rr c-C 3 H 2  HF MP2/ACCT MP2/ACCQ PBE0/ACCQ B3LYP/ACCQ B3LYP-D/ ACCQ c-C 3 H 2  HCl MP2/ACCT MP2/ACCQ PBE0/ACCQ B3LYP/ACCQ B3LYP-D/ ACCQ c-C 3 H 2  HBr MP2/ACCT MP2/ACCQ PBE0/ACCQ B3LYP/ACCQ B3LYP-D/ ACCQ Some Selected Computed Properties

Illustration of the 1D-motion of the oscillating proton along the line between the carbenic C and the heavy-atom X (X = F, Cl, Br) with a fixed separation of the heavy atom C  X bond distance at the equilibrium geometry of each c-C 3 H 2  HX complex. The motion is described approximately by an asymmetric single- (X = F) and double-well (X = Cl, Br) potential functions. MP2/aug-cc-pVTZ rigid-potential energy surface Scan

ComplexE es E ex E pl E ct E es +E ex +E pl +E ct EE BSSE  E (BSS E) c-C 3 H 2  HF c-C 3 H 2  HCl c-C 3 H 2  HBr RHF/aug-cc-pVQZ decomposed energy components (kcal mol-1) obtained from the RMP2/aug-cc-pVQZ equilibrium geometries of the c-C3H­2  HX (X = F, Cl, and Br) complexes at the RVS-SCF level

Quantum Theory of Atoms in Molecules (QTAIM) Analysis  b > 0 but small  2  b > 0 H b > 0 Non-covalent interaction  b >> 0  2  b < 0 H b < 0 Covalent interaction  b > 0 but small  2  b > 0 H b < 0 Mixed interaction: both ionic and covalent components

c-C 3 H 2 … HF Bond type bb  2  b bb VbVb GbGb |V b |/G b HbHb DI C1 C C2 H H6 F RCP C1 C2 C c-C 3 H 2 … HCl C1 C C2 H H6 Cl RCP C1 C2 C c-C 3 H 2 … HBr C1 C C2 H H6 Br RCP C1 C2 C c-C 3 H 2 C2 C C1 C RCP C1 C2 C HF H - F HCl H - Cl HBr H - Br QTAIM topological properties of c-C 3 H 2  HX (X = F, Cl, and Br) complexes obtained from the MP2/aug-cc-pVQZ level of theory.

Summary Disappointing performance of the use of small double-zeta basis sets which provide less, inaccurate, and inappropriate results so that a serious conclusion can be easily derived for any weakly bound chemical system. Therefore, it is wise to use at least a triple zeta quality basis set with at least one d- polarization function to describe the reactive intermediates (transition states) following a reactive path in a chemical reaction. The studied complexes are all red-shifted H-bonded systems. There will be one and only one minimum – the so-called global minimum – on the PES; the F-end of HX will not form any complex with H-end of c-C 3 H 2. The red-shift of the HX vibrational frequency, with concomitant bond elongation, appearance of bond paths connecting the nuclei of bonded atoms with some amount of electron density at bcps and the charge transfer effect etc. provide signature of H- bonding. It is found that c-C 3 H 2 is a strong hydrogen bond acceptor than a H-bond donor. A future experimental set up on these systems might lead to more conclusive remarks. Acknowledgements Arpita Varadwaj Gilles H. Peslherbe PBEEE Merit Postdoctoral Scholarship Award Committee, Quebec, Canada Japan Society for the Promotion of Science & Osaka Supercomputing Facilities, Japan

Conventional Signatures of Hydrogen bonding  Enhancement of dipole moment of the complex  Electron transfer from a bonding orbital of electron donor atom or group (HOMO) to the anti-bonding orbital (LUMO) of the proton donor group  Lengthening of the HY bond, with this, there is an accompanying red-shift of the HY normal mode vibration  H-bond distance < sum of van der Waals radii of non- bonded atoms ???? G. R. Desiraju, Angew Chem. Int. Ed. 50, 2011, 52 – 59

N = 1.55 Å O = 1.52 Å H = 1.05 to 1.20 Å