Effective C 2v Symmetry in the Dimethyl Ether–Acetylene Dimer Sean A. Peebles, Josh J. Newby, Michal M. Serafin, and Rebecca A. Peebles Department of Chemistry, 600 Lincoln Avenue, Eastern Illinois University, Charleston, IL USA

Introduction DME shows potential to form C–H hydrogen bonding interactions DME–HF a), DME–HCl b) and DME–CS 2 have C s symmetry and exhibit inversion splittings a) P. Ottaviani, W. Caminati, B. Velino, S. Blanco, A. Lessari, J.C. López, J.L. Alonso, ChemPhysChem, 5, (2004), 336 b) P. Ottaviani, W. Caminati, B. Velino, J.C. López, Chem. Phys. Lett., 394, (2004), Å~1.64 Å~2.89 Å S S F Cl V 2 = 59 cm -1 V 2 = 69 cm -1 V 2 = 78 cm -1

HC≡CH is a weaker H donor than HF and HCl So, what about DME–HCCH? –DME–HF and DME–HCl have C s symmetry with inversion motion of the HX molecule –Oxirane–HCCH (1) and thiirane–HCCH (2) exhibit secondary interactions between ring protons and triple bond of HCCH –H 2 O–HCCH has effective C 2v symmetry Introduction (1)(2)

Experimental Balle-Flygare Fourier-transform microwave spectrometer operating in the range 6-15 GHz DME/HCCH sample ~1.5% of each component – expanded through General Valve Series 9 valve He/Ne carrier gas at 1.5 – 2 atm backing pressure Very intense transitions; assignments confirmed by Stark effects

Spectra Only a-type transitions observed; no indication of internal rotation or inversion splittings Scaled up and down to other J transitions readily (  ~ –0.94, a near-prolate top) DME–HCCH (normal), singly substituted 13 C- DME–HCCH, DME–H 13 CCH, DME–HC 13 CH, DME–DCCD isotopic spectra were measured Second moments (although contaminated by large amplitude zero-point motions) suggest the HCCH molecule is located along the C 2 axis of DME

Fitted spectroscopic constants ParameterNormalDME- H 13 C≡CH DME- HC≡ 13 CH 13 C-DME- HC≡CH DME- DC≡CD A / MHz (17) (12) (13) (16) (14) B / MHz (18) (17) (17) (23) (17) C / MHz (17) (17) (17) (23) (17)  J / kHz –12.355(18) –11.699(17)–11.004(17)–12.93(2)–10.210(17)  JK / MHz (9)4.6580(7)4.5741(7)4.8182(9)4.4004(7)  J / kHz 5.07(4)4.90(4)4.65(4)5.20(8)4.37(4)  JJ / kHz –0.0192(7)–0.0190(7)–0.0167(7)–0.0183(15)–0.0158(7) N  rms /kHz P aa / u Å (4) (2) (3) (3) (3) P bb / u Å (4)50.042(3)50.074(3)51.428(4)50.174(3) P cc / u Å 2 –1.342(4)–1.324(3)–1.386(3)–1.477(4)–1.347(3) DME monomer: P aa = (3) u Å 2, P bb = (5) u Å 2, P cc = (3) u Å 2

Dipole moment Eight Stark lobes measured from four rotational transitions; fitted to give dipole moment :  a =  total = 1.91(10) D Dipole moment enhancement (relative to DME monomer moment of 1.31 D),  = 0.60 D Dipole moment is consistent with effective C 2v structure

DME – HCCH Structural Parameters Species fitted a) R O…H / Å All isotopomers2.0780(7) b Normal2.080(2) DME…H 13 C≡ 12 CH2.079(2) DME…H 12 C≡ 13 CH2.078(2) 13 C-DME…HC≡CH2.077(2) DME…DC≡CD2.076(2) Average2.078(2) Best guess2.08(3) Ab initio2.099 a) Fit of the parameter (B+C) for each species to the R O…H distance R O…H b a

Inertial Fit and Kraitchman coordinates (in Å) Substituted atom abc DME…H 13 C≡CH–2.138 [2.153] [0.159] [0.134] DME…HC≡ 13 CH–3.342 [3.340] [0.243] [0.000] 13 C-DME…HC≡CH1.797 [1.776] ±1.166 [1.202] [0.000] Inertial fit coordinates are given first, with Kraitchman coordinates in brackets

Ab initio Calculations MP2/ G(2d,2p) – optimization & frequency calculations gave four structures for consideration (Structures I, II, III and IV) Interaction energy (  E) corrected for BSSE a) and ZPE a) S.S. Xantheas, J. Chem. Phys., 104, (1996), I IV II III

Comparison of ab initio and experimental parameters for Structure III (C 2v ) Expt.Ab initio a) A / MHz (17)10066 B / MHz (18)1496 C / MHz (17)1324  a / D 1.91(10)2.12  a / D a) MP2/ G(2d,2p) optimization (on CP-uncorrected potential energy surface)

Ab initio structures and stabilities 1 II (C s ); 2,1, Å 2 III (C 2v ); 3,2, Å 3 IV (C s ); 4,3, Å 4 a) I (C s ); 1,4,4 a) 2.130Å a) ZPE+BSSE corrected a) Relative stabilities: Uncorrected; ZPE corrected; ZPE+BSSE corrected

Ab initio interaction energies (  E) for structures I – IV (kJ mol -1 ) IIIIIIIV (i)  E (uncorrected) –16.73–16.51–16.50–16.46 (ii)  E (+ZPE) –13.24–13.52–13.49–13.38 (iii)  E (+ZPE+BSSE) –9.62–10.16–10.15–9.87

Conclusions Ab initio calculations indicate a very flat potential energy surface and favor a geometry around the C 2v geometry BSSE and ZPE corrections are crucial to the prediction of the correct relative stabilities Experimental measurements are consistent with an effective C 2v symmetry

DME – HCF 3 Complex Structure from MP2/ G(2d,2p) optimizations 4 04 ← K = 0 lines are quartets K = 1,2 lines are doublets Each component shows additional doubling Fits of average frequencies give rotational constants close to ab initio values

Acknowledgments American Chemical Society, Petroleum Research Fund, PRF #39752-GB6 Prof. Robert Kuczkowski