rotational spectra of methylene cyclobutane and argon-methylene cyclobutane wei lin, jovan gayle...
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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