daniel p. zaleski, justin l. neill, and brooks h. pate department of chemistry, university of...
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DANIEL P. ZALESKI, JUSTIN L. NEILL, AND BROOKS H. PATE Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319,
Charlottesville, VA 22904
EWA BIALKOWSKA-JAWORSKA AND ZBIGNIEW KISIELInstitute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa,
Poland.
Structure Study of the Lactide Molecules By Chirped-Pulse FTMW Spectroscopy
The Ohio State 66th International Symposium on Molecular Spectroscopy, June 22nd, 2011.
Terminology
Referring to the chirality of the lactic acid monomers
Not the chirality of the lactide unit
3,6-Dimethyl-1,4-Dioxane-2,5-Dione
Homo-Chiral
Hetero-Chiral
G. J. van Hummel, S. Harkema, F. E. Kohn, and J. Feijen. Acta. Cryst., B38, (1982), 1679-1681.
Hetero lactic acid dimer has a small calculated dipole ~0.3 D(M052X/6-31+G(d,p)
Lactide forms from 2 lactic acid molecules, loses 2 water molecules
6-member cyclic diester
Experimental
Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate. Rev. Sci. Instrum. 79, 053103, (2008).
Originally assigned in a heated racemic lactic acid sample: 100 °C, 45 psi, neon
Acquired chirally pure commercial lactide sample: 30 psi, 150 °C
x3
24 Gs/s AWG
Center of Symmetry – Doubled Isotope IntensitiesIntensities can determine equivalent positions
Simulated isotopes going down are color-coded to left image
Isotopes
Lactide-RR
M052X/6-311++G(d,p)
EXP MP2/6-31G(d,p)
A (MHz) 2202.53576(89) 2201.0
B (MHz) 1197.00335(31) 1197.0
C (MHz) 822.34942(46) 822.3
ΔJK (kHz) 0.137(28) 0.0960
ΔK (kHz) 0.119(25) 0.1435
M052X/6-311++G(d,p): 1027 cm-1 barrier to internal rotation
EXP MP2/6-31G(d,p)
μA (D) 0 0
μB (D) 0 0
μC (D) 1.0*X 2.83
Δi (amu Å2) -37 -38
70 lines14 kHz rms
Lactide-RSEXP MP2/6-31G(d,p)
A (MHz) 2025.4060(11) 2021.1
B (MHz) 1246.64810(94) 1236.2
C (MHz) 903.0468(11) 901.1
ΔJ (kHz) 0.048(14) 0.0568
ΔJK (kHz) -0.038(54) 0.0684
ΔK (kHz) 0.192(51) 0.1231
δK (kHz) 0.070(68) 0.1579
M052X/6-311++G(d,p): 910 cm-1 (axial) and 1063 cm-1 (equatorial) barriers to internal rotation
M052X/6-311++G(d,p)
EXP MP2/6-31G(d,p)
μA (D) 0 0.26
μB (D) 0.25*X 0.58
μC (D) 1.0*X 2.52
Δi (amu Å2) -95 -98
52 lines14 kHz rms
Possible Lactide+Water Structures
a
b
EXP a b
A (MHz) 1311 1520 1366
B (MHz) 946 973 958
C (MHz) 684 826 699
Energy (cm-1) - 537 0
M052X/6-311++G(d,p)
Lactide-RR+WaterA(O+) (MHz)A(O-) (MHz)
1311.40787(40)1311.43282(56)
B(O+) (MHz)B(O-) (MHz)
946.86620(30)946.87733(56)
C(O+) (MHz)C(O-) (MHz)
684.99148(33)684.99049(79)
ΔJ(O+) (kHz) 0.3376(29)
ΔJK(O+) (kHz) -0.696(10)
ΔK(O+) (kHz) 1.513(12)
δJ(O+) (kHz) 0.1233(15)
EXP M052X/6-311++G(d,p)
μA (D) 1.0*X 2.42
μB (D) 1.0*X 2.36
μC (D) 1.25*X 2.92
Δi (amu Å2) -181 -171
215 lines9 kHz rms
Conclusions
Assigned unambiguously the homo- and hetero-chiral lactides• Including a Kraitchman substitution structure for the homo-chiral
lactide• Electronic structure theory very accurately calculates these
structures • Autofitting routines, library-free detections
Assigned a potential homo-chiral lactide-water complex• Future experiments include isotopically enriched H2
18O and D2O.
A. L. Steber, J. L. Neill, M. T. Muckle, B. H Pate, and D. F. Plusquellic. Strategies for Complex Mixture Analysis in Broadband Microwave Spectroscopy. The Ohio State 65th International Symposium on Molecular Spectroscopy. June 22, 2010.