Effective C2v 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 61920 USA

Introduction• DME shows potential to form C–H hydrogen bonding

interactions• DME–HFa), DME–HClb) and DME–CS2 have Cs

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), 336b) P. Ottaviani, W. Caminati, B. Velino, J.C. López, Chem. Phys. Lett., 394, (2004), 262

1.766 Å~1.64 Å ~2.89 Å

S

S

F Cl

V2 = 59 cm-1 V2 = 69 cm-1 V2 = 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 Cs 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

– H2O–HCCH has effective C2v 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 13C-DME–HCCH, DME–H13CCH, DME–HC13CH, DME–DCCD isotopic spectra were measured

• Second moments (although contaminated by large amplitude zero-point motions) suggest the HCCH molecule is located along the C2 axis of DME

Fitted spectroscopic constants

Parameter Normal DME-H13C≡CH

DME-HC≡13CH

13C-DME-HC≡CH

DME-DC≡CD

A / MHz 10382.5(17) 10373.6(12) 10380.1(13) 10117.6(16) 10350.4(14)

B / MHz 1535.7187(18) 1514.5348(17) 1486.0616(17) 1521.8391(23) 1448.3910(17)

C / MHz 1328.3990(17) 1312.4949(17) 1290.7486(17) 1312.7082(23) 1262.0408(17)

J / kHz –12.355(18) –11.699(17) –11.004(17) –12.93(2) –10.210(17)

JK / MHz 4.7803(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 13 14 14 12 14

rms/kHz 2.92 4.84 3.09 5.15 2.16

Paa / u Å2 330.425(4) 335.010(2) 341.466(3) 333.562(3) 350.272(3)

Pbb / u Å2 50.018(4) 50.042(3) 50.074(3) 51.428(4) 50.174(3)

Pcc / u Å2 –1.342(4) –1.324(3) –1.386(3) –1.477(4) –1.347(3)

DME monomer: Paa = 47.04660(3) u Å2, Pbb = 9.821883(5) u Å2, Pcc = 3.207326(3) u Å2

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 C2v structure

DME–HCCH Structural Parameters

Species fitted a) RO…H / Å

All isotopomers 2.0780(7) b

Normal 2.080(2)

DME…H13C≡12CH 2.079(2)

DME…H12C≡13CH 2.078(2)13C-DME…HC≡CH 2.077(2)

DME…DC≡CD 2.076(2)

Average 2.078(2)

Best guess 2.08(3)

Ab initio 2.099a) Fit of the parameter (B+C) for each species to the RO…H distance

RO…Hb

a

Inertial Fit and Kraitchman coordinates (in Å)

Substituted

atom a b c

DME…H13C≡CH –2.138[2.153]

0.000[0.159]

0.000[0.134]

DME…HC≡13CH –3.342[3.340]

0.000[0.243]

0.000[0.000]

13C-DME…HC≡CH 1.797[1.776]

±1.166[1.202]

0.000[0.000]

Inertial fit coordinates are given first, with Kraitchman coordinates in brackets

Ab initio Calculations

• MP2/6-311++G(2d,2p) – optimization & frequency calculations gave four structures for consideration (Structures I, II, III and IV)

• Interaction energy (E) corrected for BSSEa) and ZPE

a) S.S. Xantheas, J. Chem. Phys., 104, (1996), 8821.

I

IV

II

III

Comparison of ab initio and experimental

parameters for Structure III (C2v)

Expt. Ab initio a)

A / MHz 10382.5(17) 10066

B / MHz 1535.7187(18)

1496

C / MHz 1328.3990(17)

1324

a / D 1.91(10) 2.12

a / D 0.60 0.65a) MP2/6-311++G(2d,2p) optimization (on CP-uncorrected potential energy surface)

Ab initio structures and stabilitiesII (Cs); 2,1,11

2.100Å

III (C2v); 3,2,22

2.099Å

IV (Cs); 4,3,33

2.114Å

I (Cs); 1,4,44a)a)

2.130Å

a) a) Relative stabilities: Uncorrected; ZPE corrected; ZPE+BSSE correctedZPE+BSSE corrected

Ab initio interaction energies (E) for structures I – IV

(kJ mol-1)I II III IV

(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 C2v geometry

• BSSE and ZPE corrections are crucial to the prediction of the correct relative stabilities

• Experimental measurements are consistent with an effective C2v symmetry

DME–HCF3 Complex

Structure from MP2/6-311++G(2d,2p) optimizationsStructure from MP2/6-311++G(2d,2p) optimizations

404←303

7400 740374027401

• 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