1. Physique des Lasers, Atomes et Molécules
UMR 8523 CNRS – Université Lille 1, Villeneuve d’Ascq Cedex, France
60th OSU International Symposium on Molecular Spectroscopy
June 20-24, 2005
The urea-water complex observed by jet cooled Fourier-transform microwave spectroscopy and studied by ab initio calculations.
J.-R. AVILES-MORENO, A. CUISSET, T. DELEPORTE, T.R. HUET, D. PETITPREZ.

2. Content Motivations: Urea: history and structure Urea-water complex
Ab initio structure of the Urea-water complex
Experimental set-up: MB-MWFTS
Experimental spectrum:
Lines with H2O
Lines with D2O
Determination of a first experimental structure
Conclusions and outlook

3. Urea: history and structure (NH2)2C=O
One of the Nature’s simplest biomolecule.
A simplest diamide with 3 coordination sites.
Important role in pharmaceutical chemistry
Powerfull protein denaturing agent
Anti-viral agent b a

4. Urea: history and structure (NH2)2C=Ob a
Conformer Plane C2v
In the solid phase : planar a (C2v symmetry), intensive network of H-bonds
In the gas phase : nonplanar and relatively floppy
microwave spectra b
ab initio calculations
MP2 / G(d,p)
most stable conformer : anti form
1 Conformer Anti C2
2 Conformer Syn Cs b a a
E/ kJ/mol
148
192
180
5 kJ/mol
a (°) 2 1
a A S. Swaminathan et al, Acta Crystallogr., Sect B : struct. Sci. 40, 398, (1984)
b P. D. Godfrey, R. D. Brown and A. N. Hunter, J. Mol. Struct , 405, (1997)

5. UREA - WATER COMPLEX : motivation of the workb
1 Conformer H11 Anti Cs
To study the micro-solvation process of bio-molecules : possible bridge between the gas phase and the liquid phase
To understand the hydrogen bond formation.
Possible comparison between physico-chemical data coming from experiment and ab initio calculations.

6. UREA - WATER COMPLEX : Ab initio structure
Similar to formamide-water: F.J. Lovas et al J. Chem. Phys. 88(2) (1988) a b
1.85 Å
2.09 Å 1 3
0.7
3.6
130
180
230 2
Dihedral angle H11-O10-O2-C1 / °
E/ kJ/mol
P.-O. Astrand, A. Wallqvist, G. Karlström, J. Chem. Phys. 100 (1994) 1262.
Ab initio calculations at the B3LYP / aug-cc-pVTZ level of theory

7. Experimental set-up: MB-MWFTS
Spectra recorded with a pulsed-nozzle MB-MWFTS in the 6-20 GHz frequency range.
Optimization of the signal : T(K), carrier gas and pressure
Heated nozzle
T= K
Mirror
Inside resonator
Carrier gas P= 2-5 bar
Carrier gas
+ H2O
Urea powder
H2O
40 mm
Teflon

8. Experimental spectra
Large survey scan in the 6-20 GHz frequency range with a mixture of :
(14NH2)2 C=O + H2O (15NH2)2 C=O + H2O
(14NH2)2 C=O + D2O (15NH2)2 C=O + D2O
4 strong lines around 12 GHz which can be identified as the Jka,kc= b-type transitions.
Other lines around 17 GHz do not match with the urea-water complex (Jka,kc= transition for example).

9. Experimental spectra: Lines with H2O
14NH2CO14NH2– H2O
15NH2CO15NH2 – H2O
0.2 MHz
3.0 MHz
111 000
111000
~ 300MHz
15NH2CO15NH2
111 000
14NH2CO14NH2
111000
Hyperfine
structure
Splitting ~ 300MHz
T°C = 115 °C carrier gas : Ne at a total pressure of 3 bars
Tests : with and without water ; different carrier gases (Ne, Ar, He)

10. Experimental spectra: Lines with D2O
0.2 MHz
3.0 MHz
14NH2CO14NH2– D2O
15NH2CO15NH2– D2O
111000
~ 270 MHz
MHz
14NH2CO14NH2– H2O
15NH2CO15NH2 – H2O
0.2 MHz
3.0 MHz
111 000
111000
~ 300MHz
T°C = 115 °C carrier gas : Ne at a total pressure of 3 bars
Tests : with and without water ; different carrier gases (Ne, Ar, He)

11. Determination of a first experimental structure
Calculation of the water molecule’s cartesian coordinates in the principal inertia axis of urea as a function of 2 internal coordinates (ρ and φ).
Determination of the complex new inertia tensor, and diagonalisation.
Calculations of the A, B and C rotational constants for each value of ρ and φ (-40° < ρ < 40° by step of 1° and -90° < φ < 90° by step of 1° )
Comparison between (A+C)calc and (A+C)obs

12. Determination of a first experimental structure
H-bond length fixed to 1.9 Å a b ρ φ
2 internal rotations for H2O :
ρ : around the 2O-5H axis
φ : rotation of the 11H atom around the 9H-10O bond
principal inertia axis of urea

13. Determination of a first experimental structure
= | (A+C)calcd- (A+C)obsd | < 10 MHz
for 47° < ρ <50° and 40°< φ <70°
14N14N - H2O
φ (°)
ρ (°)
D (MHz)

14. Determination of a first experimental structure
(A+C)calc
(A+C)obs
ρ = 47°
φ = 55°
14N14N - H2O
12285
15N15N - H2O
11987
11977
ρ = 49°
14N14N - D2O
11770
11741
15N15N - D2O
11489
11475
r1=1.87Å
r2=1.87Å
=90° 
* Fixed value

15. Conclusions and outlook
First experimental observation of the urea-water complex.
Searching for more urea-water complex lines.
Possibility of a large amplitude motion of the H11 atom.
Change of the H-bond length when going from H2O to D2O.
In progress: analysis of the hyperfine structure

16. Experimental set-up: MB-MWFTS
n  n0 t
detection
source
polar gas e
Molecules n0 n
Source of microwave pulse
(2-20 GHz)
Matter-light interaction
Inside a Pérot-Fabry resonator
Polarization of the polar molecules ;
Rotational cooling
Detection and recording of the signal
Emitted by the molecules
As a function of time
Fourier transform of the transient signal
 Frequency analysis
Application rule : maximum polarization for a p/2 pulse, i.e.
Physical parameters :
m : permanent electric dipole moment
e : amplitude of the microwave field
t : length of the microwave pulse

17. Experimental set-up: MB-MWFTS
Gas
mixture
Vacuum-tight
rotation
transition
and
step by step
motor
Gaussian envelop
(Ws= 42 mm at 12 GHz) MW
pulse
detection
pump
600 mm<d< 650 mm
R=800 mm
Resonant cavity and pulsed supersonic beam
Spectral range :
6 – 20 GHz
Sensitivity :
10-11 cm-1
Resolution : 10 kHz
Accuracy : 2.4 kHz
Rot. temp. :  4 K
Pressures :
Carrier gas: 1-3 bars
Molecules: 10-2 bar
Secondary pumping
Labview interface
Scan : 1GHz/12h
Heated
nozzle
363 K
Benzamide powder
1.5 bar Ar
Carrier
gaz
Cavity

18. Transition rotationnelle Dédoublement Doppler
Interrupteur
rapide t
Impulsion microonde de 1 à 2 ms
Synthétiseur
2-20 GHz
Cavité PF
Amplificateur
A/D
FI = 30 MHz
Mélangeur
Filtre passe-bande
Amplificateur RF
Convertisseur A/D
ns+30 MHz
Transition rotationnelle
Dédoublement Doppler