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

Heated lactide spectrum

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

Boat vs Chair

Chair is not a minimumBoat is the detected structure

M052X/6-311++G(d,p)

O-

O+

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.

Acknowledgments

Award Number CHE-0960074