Article pubs.acs.org/JPCA
Microwave Spectrum and Intramolecular Hydrogen Bonding of 2‑Isocyanoethanol (HOCH2CH2NC) Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡ †
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Blindern, NO-0315 Oslo, Norway ‡ Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France S Supporting Information *
ABSTRACT: The microwave spectrum of 2-isocyanoethanol (HOCH2CH2NC) has been investigated in the 12−120 GHz spectral range. The assignment of this spectrum was severely complicated by the rapid transformation of 2-isocyanoethanol into its isomer 2-oxazoline, which has a rich and strong spectrum. This process appeared both in a gold-plated microwave cell and in a brass cell and is presumed to be catalyzed by metals or traces of base. The spectrum of one conformer was ultimately assigned. This form is stabilized by an intramolecular hydrogen bond between the hydroxyl group and the isocyano group and is the first gas-phase study ever of this kind of hydrogen bonding. The distance between the hydrogen atom of the hydroxyl group and the nitrogen and carbon atoms are as long as 256 and 298 pm, respectively, indicating that covalent contribution to the hydrogen bond is minimal. Electrostatic forces are much more important because the O−H and NC bonds are almost parallel and the corresponding bond moments are practically antiparallel. The microwave work has been augmented by quantum chemical calculations at the CCSD(T)/cc-pVTZ and MP2/cc-pVTZ levels of theory. Results of these calculations are generally in good agreement with experimental findings.
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INTRODUCTION The Oslo laboratory has for a long time been interested in intramolecular hydrogen bonding and used microwave (MW) spectroscopy often augmented with quantum chemical calculations to investigate a wide variety of internal hydrogen bonds.1−4 Recent studies have shown that the π-electrons of the triple bonds of the alkynyl (R−CC−R′) and the nitrile (R−CN) functional groups are weak proton acceptors.5 Examples of alkynes with intramolecular hydrogen bonds investigated by MW spectroscopy include propargyl alcohol (HOCH2CCH),6 3-butyn-2-ol (H3CCH(OH)CCH),7 3butyn-1-ol (HOCH 2 CH 2 CCH), 8 − 1 2 4-pentyn-1-ol (HOCH2CH2CH2CCH),13 propargyl thiol (HSCH2C CH),14 3-butyne-1-thiol (HSCH2CH2CCH),15 propargyl selenol (HSeCH2CCH),16 3-butyne-1-selenol (HSeCH2CH2CCH),17 propargyl amine (H2NCH2C CH),18 N-methylpropargyl amine (H3C(NH)CH2CCH),19 and 1-amino-3-butyne (H2NCH2CH2CCH).20 Similarly, intramolecular hydrogen bonding has been reported for a number of nitriles comprising hydroxyacetonitrile (HOCH2CN),21 lactonitrile (H3CCH(OH)CN),22 3hydroxypropanenitrile (HOCH2CH2CN),23 3-mercaptopropionitrile (HSCH2CH2CN),24 Z-3-mercapto-2-propenenitrile (HSCHCHCN),25 aminoacetonitrile (H2NCH2C N),26,27 3-aminopropionitrile (H2NCH2CH2CN),28 (Nmethylamino)ethanenitrile (H3C−NH−CH2CN),29 2-aminopropionitrile (H3C(NH2)CHCN),30 Z-3-amino-2-propenenitrile (H2NCHCHCN),31 and 3-phosphinopropionitrile (H2PCH2CH2CN).32 © 2014 American Chemical Society
In contrast to these many investigations of internal hydrogen bonding with the π-electrons of the triple bonds in alkynes and nitriles, no gas-phase studies of this interaction have been reported for isonitriles (R−NC), the third functional group possessing a triple bond. The present study of 2-isocyanoethanol (HOCH2CH2NC) by MW spectroscopy is therefore the first gas-phase study ever of the ability of the isocyanide group to participate in intramolecular hydrogen bonding. Isocyanides have an interesting but relatively little explored chemistry.33−35 Our two laboratories have therefore started to investigate the physical properties of several isocyanides and already reported the MW spectra of allenyl isocyanide (H2C CCHNC),36 2-fluoroethyl isocyanide (FCH2CH2N C),37 2-chloroethyl isocyanide (ClCH2CH2NC),38 E- and Z-1-propenylisocyanide (CH3CHCHNC),39 cyclopropylmethyl isocyanide (C3H5CH2NC),40 4-isocyano-1-butyne (HCCCH2CH2NC),41 and 4-isocyano-1-butene (H2C CHCH2CH2NC).42 The present study of 2-isocyanoethanol is an extension of these investigations focusing on the possible intramolecular hydrogen bonding of this compound. A model of 2-isocyanoethanol is shown Figure 1. Rotational isomerism is possible about the C−C and C−O bonds resulting in five possible conformers, denoted Conformer I−V. Atom numbering is indicated on Conformer I. The O5−C1−C2−N3 and C2−C1−O5−H10 chains of atoms can conveniently be Received: March 4, 2014 Revised: April 2, 2014 Published: April 2, 2014 3120
dx.doi.org/10.1021/jp502212n | J. Phys. Chem. A 2014, 118, 3120−3127
The Journal of Physical Chemistry A
Article
Scheme 1. Synthesis of 2-Isocyanoethanola
H NMR (CDCl3, 400 MHz) δ 3.50 (tt, 3H, 3JHH = 5.5 Hz, 2JNHqd = 1.8 Hz, CH2N); 3.60 (s brd, 1H, OH); 3.75 (tt, 1H, 3JHH = 5.5 Hz, 3 JNHqd = 2.4 Hz, CH2−O). 13C NMR (CDCl3, 100 MHz) δ 44.6 (1JCH = 144.5 Hz (t), 1JCNqd = 6.9 Hz (t), CH2N); 60.0 (1JCH = 145.2 Hz (t), CH2−O); 155.6 (1JCNqd = 6.2 Hz (t), NC). qd = quadrupolar coupling. a1
instead of gaseous phosgene. The compound was purified by distillation in vacuo of about 1 g to obtain the product with a purity around 90%. The impurities were mainly unreacted HOCH2CH2NHCHO and a little 2-oxazoline (
