John Mullaney Newcastle University Geometry of an Isolated Dimer of Imidazole Characterised by Rotational Spectroscopy and Ab Initio Calculations John Mullaney Newcastle University

Imidazole Aromatic heterocycle Pyrrolic and pyridinic nitrogen atoms Salts have applications as carbene precursors and ionic liquids Part of the amino acid histidine, the hormone histamine, nucleobases adenine and guanine and vitamin B12 H bonds as a donor or acceptor

Crystal structure of imidazole Series of N-H---N H bonding interactions H bonded ribbons of imidazole stack H bonding structure present at 20 and -150 oC B. M. Craven et al., Acta Cryst., 33, 1977, 2585

IR spectroscopy of IM/IMD Used He nanodroplets as a matrix for the monomer, dimer and water complexes - Larger droplets favour formation of the IMD and higher order complexes H bonding interaction (a) broad stretching frequency determined ~ 3240 cm-1 N-H stretching frequency (b) also determined ~ 3510 cm-1 a b M. Y. Choi and R. E. Miller, J. Phys. Chem. A, 110, 2006, 9344

Chirped-pulse Fourier transform microwave spectrometer (CP-FTMW) - Argon carrier gas, 6 bar - Expansion in supersonic jet - Gas sample passes over rod - Nd:YAG laser ablates composite rod Vacuum chamber N. R. Walker et al., Phys. Chem. Chem. Phys., 16, 2014, 25221

Rod composition Initial experiment aimed to form metal and imidazole containing complexes in gas phase Molar ratios of 1:1  16:1 of copper:imidazole were tested (5 & 13 mm diameter rods) Comparable signals were measured with rods of activated charcoal Imidazole signals significantly weaker if using just imidazole

250 C3H4N2 200 150 Intensity (µV) 100 CH3CN 50 HCCCN

Intensity (µV) (C3H4N2)2 (C3H4N2)2 J = 19 - 18 J = 20 - 19 250 200 150 100 50

Fragmentation products - Species formed and observed as a result of laser ablation of imidazole - MeCN signal strong enough to observe 13CH3CN Vibrationally excited states observed Species observed include: CH3CN, CH3CH2CN, CH2CHCN, HCCCN, NH2CH2CN, HC5N, HC7N, and CH3C3N

(C3H4N2)2 (C3H4N2)(C3H3DN2) (C3H3DN2)2 (C3D4N2)2 A0 / MHz 4800(25)a   (C3H4N2)2 (C3H4N2)(C3H3DN2) (C3H3DN2)2 (C3D4N2)2 A0 / MHz 4800(25)a 4710(50) 4680(53) 3970(16) B0 / MHz 457.53869(27) 457.32707(37) 449.86071(31) 424.53190(40) C0 / MHz 456.03871(26) 455.84958(38) 448.01603(31) 422.81625(39) DJK / kHz 8.3740(54) 8.299(15) 7.828(14) 7.311(11) [DJ  102] / kHz 6.907(15) 6.773(22) 6.952(25) 5.926(18) d1 / Hz 2.63(22) 2.96(29) 3.39(28) 2.10(28) D3 / Hz 3.79(28) 4.4(7) - 0.56(25) N 63 36 39 43 σr.m.s. / kHz 6.4 7.9 6.9 8.0 a Numbers in parentheses are one standard deviation in units of the last significant figure. G. Winnewisser, J. Chem. Phys., 57, 1972, 1803. G. Winnewisser, J. Chem. Phys., 56, 1972, 2944.

Minimum of 6 parameters to describe the geometry are required RCM, θ and  describe relative positions of the monomer units ,  and  define the orientation of the 2nd monomer with respect to the 1st   tilting   turning   twisting   Exp. (r0) CCSD(T)(F12*)/VDZ (re) RCM / Å 5.2754(3) 5.297  /  [90] 90.0  /  103.4(11) 102.2  /   /  120.0(12) 115.1  /  87.91(8) r(HN) / Å 1.9343(3) 1.947 (NHN) /  175.9(11) 179.6

Structure - <(NH...N) shows slight twisting   Exp. (r0) CCSD(T)(F12*)/VDZ (re) RCM / Å 5.2754(3) 5.297 r(HN) / Å 1.9343(3) 1.947 (NHN) /  175.9(11) 179.6 - <(NH...N) shows slight twisting Good agreement with calculations Not all structural parameters were able to be determined from the 8 measured rotational constants Experimental geometry = Small spheres CCSD(T)(F12*)/cc-pVDZ-F12 calculated geometry = Large spheres

Conclusions Measured rotational spectrum of the imidazole dimer free of any solvent effects H bonding interaction present in imidazole dimer Beneficial to mix organic precursors with an inert matrix with respect to vaporisation

Acknowledgements Newcastle University Nick Walker Dror Bittner Chris Medcraft Simon Doherty University of Manitoba Susanna Stephens Argonne National Laboratory Daniel Zaleski University of Bristol David Tew Tony Legon