Rotational Spectra of Methylene Cyclobutane and Argon-Methylene Cyclobutane

Wei Lin, Jovan Gayle Wallace Pringle, Stewart E. Novick

Department of ChemistryWesleyan University

Middletown, CT

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

Introduction Ring Strain and

Torsional Strain give rise to double minimum potential and large amplitude, low frequency “butterfly-like” inversion motion.

Potential Function for mcb1

160cm-1

1L.H Scharpen and V.W Laurie, J . Chem. Phys. 49, 3041-3049 (1968)

25cm-1

1.12cm-

1

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

Observed Spectra and Analysis 34 a, c - type transitions observed and

assigned for methylene cyclobutane

Full heavy atom substitution structure determined from 13C in natural abundance

Transitions assigned combined with those from previous work and fit to 13 spectroscopic constants including those for coriolis coupling, Fac, and energy spacing, ΔE01, between the two levels.

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

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, Fac, to be directly and more precisely determined

Spectroscopic Constants

H01=(Fac + F’acJ(J+1))(PaPc + PcPa)

v = 0 v = 1 v = 0 v = 1 v = 0 v = 1A/MHz 10372.74(5) 10365.406(7) 10372.93(4) 10365.54(5) 10372.824(3) 10365.391(8)B/MHz 4604.3109(7) 4606.067(7) 4604.307(2) 4606.083(2) 4604.3079(4) 4606.0792(5)

C/MHz 3462.7171(10 3465.770(6) 3462.716(2) 3465.755(2) 3462.7198(4) 3465.7589(4)D

J/kHz 1.12(2) 0.87(8) 1.02(1) 0.99(1) 1.050(8) 0.996(8)D

JK/kHz -0.6(2) 1.6(9) -0.16(5) 1.50(6) -0.28(5) 1.43(4)D

K/kHz -10(10) -53(40) 11(2) 5(2) 10.3(8) 4(1)d

J/kHz 0.03(2) 0.08(2) 0.032(4) 0.088(4) 0.029(3) 0.082(3)d

k/kHz 3.2(3) -4(3) 1.4(2) 1.2(2) 1.8(1) 1.5(1)DE01/MHFac/MHz

F’ac/MHzN

s /kHz

This work Previous work Combined fit

33615.53(4) 33616.2(3) 33615.597(6)140.154(7) 140.148(4) 140.1459(9)

-1.5(4) -0.99(3) -0.96(2)34 125 1413 37 40

Substitution Analysis Rotational constants

for singly 13C substituted isotopomers allowed calculations of the positions of the 12C atoms, using Kraitchman analysis

DE01 undergoes small decreases for 13C substituted isotopomers

γβ’

β

α

m

Conformation of MCB Methylene C atom bent 11° away from

plane containing β and α C atom

γ

β’

β

α

m

Methylene cyclobutane

Isobutene

β’

β

α m

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 13C singly substituted Ar-mcb complex in natural abundance

Spectroscopic Constants12C α-13C β-13C γ-13C m-13C

A/MHz 3484.0660(4) 3474.9887(5) 3452.4303(4) 3431.047(1) 3397.5233(3)B/MHz 1308.0501(4) 1301.8092(7) 1296.7781(6) 1302.848(2) 1303.9513(4)C/MHz 1127.9153(3) 1122.2994(6) 1122.1100(5) 1118.390(2) 1115.5422(3)D

J/kHz 5.003(3) 4.956(7) 4.926(5) 4.91(2) 4.900(4)D

JK/kHz 0.03874(2) 0.03834(3) 0.03938(3) 0.03716(7) 0.03719(2)D

K/kHz -0.03981(4) -0.03944(5) -0.04028(3) -0.0380(1) -0.03855(2)d

J/kHz 0.764(2) 0.754(2) 0.738(2) 0.770(6) 0.779(2)d

K/kHz -4.2(1) -4.0(3) -5.3(2) -4.1(7) -3.2(1)f

K/kHz 0.063(2)

N 143 18 25 19 22s /kHz 4 1 2 4 1

0.063a

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

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’γ

m

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

Argon Position in Other Ring Complexes

a b cAr-methylene cyclobutane 0.11 0.51 3.62Ar-cyclobutanone 0.23 0.55 3.48Ar-thietane 0 0.57 3.79

Argon Position in Other Ring Complexes

a b cAr-oxetane 0.67 0.14 3.5Ar-chlorocyclobutane 1.267 2.824 2.517