Equilibrium Molecular Structure and Spectroscopic Parameters of Methyl Carbamate J. Demaison, A. G. Császár, V. Szalay, I. Kleiner, H. Møllendal

GOALS OF THIS STUDY: -check predictive power of different ab initio methods for larger molecules of biological interest. -structure is planar or not? -Methyl carbamate (H 2 NC(O)OCH 3 ) : isomer of the simplest aminoacid, glycine (H 2 NCH 2 COOH). Biological effects and pharmaceutical applications. -possible detection in interstellar space : might be more abundant than glycine and rotational spectrum more intense (bigger dipole moment).

-Previous works : Microwave 1) K-M. Marstokk and H. Møllendal, Acta Chem. Scand. 53 (1999) 79-84: a) Only one conformer found (syn conformation). b)Approximate values of the barrier to internal rotation of the methyl group, the 14 N quadrupole coupling constants and dipole moments. c) ab initio geometrical structure but no accurate centrifugal distortion constants.

[2] Bakri, J. Demaison, I. Kleiner, L. Margulès, H. Møllendal, D. Petitprez, G. Wlodarczak, J. Mol. Spectrosc. 215 (2002) FT MW and millimeterwave: 415 A-type and 98 E type transitions in v t = 0 ground torsional state -Accurate values for 14 N quadrupole coupling constants and centrifugal distortion constants for v t =0. -Different ab initio methods (Gaussian 98) -  syn conformation of methyl carbamate significantly more stable than the most stable isomer (Ip) of glycine HOWEVER! -Contrary to glycine, there is NO ACCURATE STRUCTURE available for methyl carbamate. -Need for a MORE COMPLETE EXPERIMENTAL WORK

CAN WE CALCULATE SPECTROSCOPIC PARAMETERS AT REASONABLE COST FOR LARGER MOLECULES OF BIOLOGICAL INTEREST?

Equilibrium B3LYP/VTZ values apparently closest to the Experimental ground states. Second best RHF/3-21G* (computation Less than 1 min!) BUT Equilibrium computed constants from ab initio ARE NOT ground state Values!!! Corrections from force field calculation using MP2/6-31G*: A e – A 0 = 82.1 MHz B e – B 0 = 39.6 C e – C 0 = 30.8 So the agreement with B3LYP values can be an accident… However: the force field correction MP2/6-31G* is also an approximation: -small amplitude vibrations (not true) -not the best method -ab initio structure are found to be non-planar…

Observed and calculated vibrational frequencies for methyl carbamate. exp. HF/6-31G* B3LYP/VTZ Assignmentcm -1 e – c in %e – c in % a' species 1 NH 2 antisym stretch3551  0.1  NH 2 sym stretch3435  0.1  CH 3 antisym stretch2957  1.1  CH 3 sym stretch2874  1.1  C=O stretch1747  2.2  NH 2 bending1583 SCALING  0.9 SCALING CH 3 antisym deform1460 FACTOR  1.0 FACTOR  CH 3 sym deform1369  6.8  C-N stretch a  b OC-O stretch1195  NH 2 rock1108  CH 3 rock1075  H 3 C-O stretch C=O rock OCN deform COC deform a" species 17 CH 3 antisym stretch  CH 3 antisym deform1447  1.3  CH 3 rock1071  8.4  NH 2 wag C=O wag NH 2 inversion203 23? 24? a [Pople et al 1981]. b for all CH stretchings and for all others [Martin et al 1996].

Observed and calculated quartic centrifugal distortion constants for methyl carbamate. Exp. (kHz) a Calc.B3LYP/ VTZ (I) b Torsion contr. c Calc. e  c(II) (%) ∆j∆j (7) ∆ jk (29)  15.6 ∆k∆k (22)  1.9 jj (3) kk (33)  3.3 a For the A component of the internal rotation doublet. b "Unperturbed" constant. c Contribution of the internal rotation. Calculated with F = GHz, s = 28, and a = [Hersbach].

Computed and experimental dipole moment components of methyl carbamate.

Observed and calculated 14 N nuclear quadrupole coupling constants for methyl carbamate Exp. (I) [Martskok Mollendal 1999] MHz Exp. (II). [Bakri et al 2002] MHzCalc. MHz e – c(II) (%) Calc. MHz e – c(II) (%) eQq aa 1.52 (27) (7) eQq bb 3.51 (20) (8) eQq cc (33) (8) HF/VTZB3LYP/AVTZ

IS IT POSSIBLE TO CALCULATE ab initio AN ACCURATE TORSIONAL BARRIER AT A REASONABLE COST FOR METHYL CARBAMATE?

Planarity of the C(O)NH 2 linkage: -The peptide linkage is generally assumed to have a planar structure due to the contribution of a resonance structure O - CX=N + HY, which induces a partial double bond character of the C–N bond. However, the contribution of each resonance structure can be changed with interactions with the environment. -non planarity of some peptide linkages attributed to a low potential methyl top barrier?

Table 5. Structure of carbamic acid (distances in Å and angles in degree). CLEARLY NON PLANAR BUT the energy difference between planar and non planar is small: about cm -1

-ALL ab initio optimizations indicate that the amide group is non planar (difference between planar and non planar is 53 cm -1 CCSD(T)/V(T,D)Z in contradiction with experimental results (  c is zero) WHAT’s GOING ON? MC behaves like other molecules containing the amino group. small barrier between planar and non planar and the ground torsional state is above this barrier.

Kydd and Rauk, J. Mol. Struct. 1981

Brown, Godfrey, Kleibomer JMS 124, Acetamide JMS 440,

cyanamide vinylamine CH 2 CHNH 2 JMS 114, JMS 124,

CONCLUSIONS -ab initio methods can give us information. Different methods/basis set should be tried before conclusions. -near-planarity in methyl carbamate should be investigated further. Need for new experimental MW data (Kharkov) and FIR high resolution. This will also provide a line list for astrophysical purpose.

propianamide

Ground-state inertial defects ? For a molecule with a plane of symmetry and two out-of-plan H : = I 0 a +I 0 b -I 0 c = m HH d 2 HH –  v d HH = Å -  computed ground-state inertial defect (B3LYP/AVTZ) m HH d 2 HH = uÅ 2.  exp = uÅ 2  the vibrational contribution  v = uÅ 2. Such a small positive contribution is compatible with a planar structure but at the other hand the Computed G-S inertial defects for the planar and non planar forms are very close (3.171 and uÅ 2 )