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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 250
C3H4N2
200
150
Intensity (µV)
100
CH3CN 50
HCCCN

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

9. 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

10. (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
(27)
(37)
(31)
(40)
C0 / MHz
(26)
(38)
(31)
(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.

11. 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

12. 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

13. 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

14. 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