1. Rotational Spectra of Methylene Cyclobutane and Argon-Methylene Cyclobutane Wei Lin, Jovan Gayle Wallace Pringle, Stewart E. Novick Department of Chemistry Wesleyan University Middletown, CT

2. Introduction Methylene Cyclobutane first studied in 1968 by Sharpen and Laurie using conventional microwave spectroscopy Mid-infrared spectra examined by Malloy et al in 1970 Raman spectra investigated in 1972 by Durig et al First eight excited states examined in the millimeter wave region by Charro et al in 1993

3. Introduction Ring Strain and Torsional Strain give rise to double minimum potential and large amplitude, low frequency “butterfly-like” inversion motion. Potential Function for mcb 1 160cm -1 1 L.H Scharpen and V.W Laurie, J. Chem. Phys. 49, 3041-3049 (1968) 25cm -1 1.12cm -1

4. Experimental Pulsed – jet Fourier Transform Microwave Spectrometer used Frequencies from 6- 26GHz applied Mixture used was 0.5% methylene cyclobutane in Argon at a backing pressure of 0.2 atm

5. Observed Spectra and Analysis 34 a, c - type transitions observed and assigned for methylene cyclobutane Full heavy atom substitution structure determined from 13 C in natural abundance Transitions assigned combined with those from previous work and fit to 13 spectroscopic constants including those for coriolis coupling, F ac, and energy spacing, ΔE 01, between the two levels.

6. Ring Puckering Transitions in methylene cyclobutane Pure rotational transitions - connect rotational levels within the ground state and connect rotational levels within the first excited state Ring puckering rotational transitions – connect rotational levels in the ground state with those in the first excited state 160cm -1 v=0 v=1 1.12cm -1

7. Ring Puckering Transitions 10 c-type, Δv=±1 transitions measured These transitions allowed the constants for the vibrational energy spacing constant, ΔE, and coriolis coupling constant, F ac, to be directly and more precisely determined

8. Spectroscopic Constants H 01 =(F ac + F’ ac J(J+1))(P a P c + P c P a )

9. Substitution Analysis Rotational constants for singly 13 C substituted isotopomers allowed calculations of the positions of the 12 C atoms, using Kraitchman analysis  E 01 undergoes small decreases for 13 C substituted isotopomers γ β’β’ β α m

10. Conformation of MCB Methylene C atom bent 11° away from plane containing β and α C atom γ β’β’ β α m Methylene cyclobutane Isobutene β’β’ β αm 11°

11. Ar – Methylene Cyclobutane 143 a, b, c - type transitions assigned for Ar- methylene cyclobutane Fit to 3 rotational constants, 5 quartic centrifugal distortion constants, 1 sextic centrifugal distortion constant Approximately 20 b-type transitions measured for each 13 C singly substituted Ar- mcb complex in natural abundance

12. Spectroscopic Constants

13. Substitution Analysis Position of Argon calculated using Kraitchman substitution analysis Coordinates of Argon in principal axis system of methylene cyclobutane: a = 0.11 Å, b = 0.51 Å, c = 3.62 Å

14. Wide Amplitude Motion of Ar Only 4 isotopic structures were acquired for the complex, for one of these, the intensity of the transitions was twice that of the other three This indicated that 2 C atoms are equivalent, meaning that the equilibrium position of the Ar atom is on the plane of symmetry α β β’β’ γ m

15. Position of Argon in Ar-methylene cyclobutane Ring slightly bent, argon atom in endo position van der Waals bonding of Argon to ring quenches inversion motion Argon undergoes large amplitude motion across ring

16. Argon Position in Other Ring Complexes