Chirped-pulse, FTMW spectroscopy of the lactic acid-H 2 O system Zbigniew Kisiel, a Ewa Białkowska-Jaworska, a Daniel P. Zaleski, b Justin L. Neill, b.

Chirped-pulse, FTMW spectroscopy of the lactic acid-H2O system

Zbigniew Kisiel,a Ewa Białkowska-Jaworska,a

Daniel P. Zaleski,b Justin L. Neill,b Amanda L.Steber,b Brooks H. Pate b

aInstitute of Physics, Polish Academy of Sciences,Warszawa, Poland

cDepartment of Chemistry, University of Virginia,

Charlottesville, Virginia, USA

66th OSU International Symposium on Molecular Spectroscopy WH1066th OSU International Symposium on Molecular Spectroscopy WH10

Lactic acid primer:

SsC (most stable conformer) a = 1.44 D b =

1.83 D

GskC +8.0kJ/mol 3.6% @298K

G’sk’C +8.6 2.9%

AaT +10.0 1.6% Gauche

matrix isolation ir + ab-initio, Borba,Gomez-Zavaglia,Lapinski,Fausto, less stable conformers PCCP 6, 2101 (2004)

MMW of SsC Pszczolkowski et al., J.Mol.Spectrosc. 234, 106 (2005)

ab-initio, LA+H2O Sadlej et al., PCCP 8, 101 (2006)ab-initio, LA+nH2O Smaga,Sadlej, J.Phys.Chem.A 114,4427(2010)

ir + ab-initio, LA aggregation Losada,Trang,Xu, J.Chem.Phys. 128, 014508 (2008)

Chirped-pulse Fourier transform microwave spectrometer with supersonic expansion:

Developed in the Pate group, U of. Virginia, USA:

Up to 10 GHz in a single shot !

• Rev.Sci.Instrum. 79, 053103 (2008)

Enhancements:

- Several nozzles- Multiple chirps on

one gas pulse

Analysis tools: AABS package + SPFIT/SPCAT:

Line blanking

Chirped pulse spectrum of Lactic acid (+H2O):

LA, SsC

LA, AaT

LA+H2O

LA+(H2O)2

(H2O)2

Obs.

Calc.

LA conformers observable in supersonic expansion:

SsC a = 1.49 D b =

1.92 D

AaT a = 5.05 D b = 0.60 D

DE = +10.0 kJ/mol 1.6% @298K

B3LYP/6-311++G(d,p)

Relative intensities of SsC and AaT transitions:

Obs.

Calc.

212 ← 111 202 ← 101 211 ← 110

1 MHz

SsC AaT SsC AaT SsC AaT

Calculation with: a = 1.44 D for SsCa = 5.05 D for

AaT and AaT = 2% of SsC

AaT

Constants for the lactic acid monomers:

PC-GAMESS, B3LYP/6-311++G(d,p)

quartics FCONV + VIBCA

20 aR-type lines7 bR- and bQ-type

sfit = 12.1 kHz

SsC

Ab-initio predictions for LA + H2O:Smaga,Sadlej, J.Phys.Chem.A 114,4427(2010)MP2/aug-cc-pVDZ

Ia Ib

Ic

- 31.6

- 22.9Most stable form with total binding energy

- 40.9 kJ/mol

The J = 4 3 aR-type transition of LA+H2O:

Obs.

Calc.T=0.5K

Ka = 1 0 2 2 1

1 MHz

Ab-initio predictions for LA+ 2 x H2O:

Smaga,Sadlej, J.Phys.Chem.A 114,4427(2010)

IaIb IaIc

dw-I

- 71.8

- 64.4

- 66.0

In fact:

- 87.8

The J = 7 6 transition of LA+2H2O:

Ka = 1 0 2 5 4 4 3 3 2 1

Calc. T=1K

a-type

7 07

61

6

7 17

60

6

b-type

*

The J = 10 9 aR-type transition of LA+3H2O:

Ka = 1 0 2 6 55 3 4 4 1 3 2

Calc. T=1K

LA+3H2OLA+2H2O

Constants for the lactic acid+water clusters:

PC-GAMESS, MP2/aug-cc-pVDZ

quartics FCONV + VIBCA

aR- and bR-type lines in all casessometimes also bQ-types

LA+H2O

Large cycle

BSSE calculated binding energies (kJ/mol):

- 87.8

- 121.9

- 41.8

- 15.2

- 14.2- 32.1

- 13.8

- 36.0

- 18.9

- 14.6

3-body = 22%

≥ 3-body = 38%

Binding energies and H-bond lengths:

2.621 Å

2.798 Å

2.652 Å

2.796 Å2.710 Å

2.809 Å

Comparison with water clusters:

2.921 Å 2.710 Å

2.706 Å

2.720 Å

(H2O)3 2.803 Å

(H2O)4 2.748 Å

all at MP2/aug-cc-pVDZ

The rotational spectrum of the less stable AaT conformer of lactic acid was assigned for the first time.

Supersonic expansion SsC : AaT ratio found to be close to the room temperature value.

Rotational spectra of LA+H2O, LA+2H2O, LA+3H2O assigned (either classically or using the “3-line” autofitting program (TC10, 65th OSU Symposium).

The three clusters constitute a series of successively larger water chains bound in a cycle to the carboxylic group of the lactic acid .

The rotational constants, centrifugal distortion constants, and electric dipole moment components are in good semi-quantitative agreement with results of ab initio calculations.

Attempts to observe the lactic acid dimers were eventually successful in that chemical dimers = lactides were identified in a heated sample (stay for the next talk = WH11)

SUMMARY: