Article pubs.acs.org/JPCA
Experimental and Theoretical Investigation of the Pyrolysis Products of Iminodiacetonitrile, (NCCH2)2NH Osman I. Osman* Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Chemistry Department, Faculty of Science, University of Khartoum, P.O. Box 321, Khartoum 11111, Sudan School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom S Supporting Information *
ABSTRACT: The gas-phase FTIR study of the pyrolysis products of iminodiacetonitrile, (NCCH2)2NH has revealed the existence of C-cyanomethanimine, NCCHNH, and ketenimine, CH2CNH. The former has two isomers: Z and E; while the later readily taumerizes to acetonitrile, CH3CN. A trapping/revaporization system has been used to purify C-cyanomethanimine. The analysis of the rotational structures of the IR medium resolution C-type CNH bend, ν6, and CN torsional mode, ν10, has led to a conformational characterization of these isomers. The Z-isomer was shown to be the major product. This conjecture was supported by ab initio MO calculations that confirmed the relative total energy stability of the Z-isomer over its E-counterpart by 0.173 to 2.326 kJ/mol. The K values indicated that the equilibrium concentration of Z-C-cyanomethanimine amounts to up to three times that of E-C-cyanomethanimine. A further investigation using NBO technique proved the predilection of the Z-isomer. In addition it relates its provenance of preference to the remote nN6 → σ*C4−N5 interaction that stabilized it by 1.10 kcal/mol. A thorough theoretical investigation of the tautomerization reaction between ketenimine and acetonitrile will be published in a separate contribution.
1. INTRODUCTION The interest in the various possible compounds with the CN moiety and/or the hydrogen cyanide dimers has recently drawn the attention of many researchers.1−3 This stems from their possible role as being intermediates in the prebiotic synthesis of purines and proteins.4 They may also exist in space. In fact, hydrogen cyanide monomer is relatively abundant in many parts of the interstellar medium.5 Thus, it is a very important chemical entity in the field of astrobiology that deals with the production of molecules vital in the development of life.6 Terrestrially, HCN is an important precursor of biologically significant molecules such as amino acids.7 Moser et al.8 have reported the possible existence of the biradical aminocyanocarbene (NH2CCN) when they pyrolyzed azidoacetonitrile, NCCH2N3. Theoretical calculations9 have shown that C-cyanomethanimine, NCCH NH, is more stable than the aminocarbene. Clemmons et al.10 have predicted C-cyanomethanimine, NCCHNH, to be the most stable HCN dimer compared to HCN··· HCN and N-cyanomethanimine, CH2NCN. The linear van der Waals dimer HCN···HCN has been produced by isentropic expansion of HCN from a system of nozzles and identified by microwave spectroscopy.11 Its infrared absorption spectrum was reported by Pacansky.12 Johansson et al.13 and Kollman et al.14 have performed thorough theoretical calculations to monitor its geometry and dimerization energy. In 1971, it was detected in various region of space by Snyder and Buhl.15 © XXXX American Chemical Society
The pyrolysis of dimethyl cyanamide or trimethylenetetrazole have yielded N-cyanomethanimine, CH2NCN, which was characterized by millimeter wave,16 infrared,17 and photoelectron18 techniques. Little data have been collected that coincided with the rotational transitions of C-cyanomethanimine, NCCHNH, but insufficient to indicate its unequivocal existence in space.19 Using infrared spectroscopy, Lorencak et al.20 have characterized Z- and E-isomers of cyanomethanimine in an argon matrix; while Hamada et al.,21 have indentified C-cyanomethanimine, NCCHNH, in the gas-phase pyrolysis products of dimethylcyanamide. Independently, about the same time we managed to record the infrared spectrum of C-cyanomethanimine through the thermolysis of iminodiacetonitrile.22 The unequivocal assignments of the fundamental frequencies in both studies were hampered by impurities. Takano et al.23 reported the microwave spectra and energy difference of E and Z C-cyanomethanimine. They showed that the Z-isomer is ca. 0.615 ± 0.143 kcal/mol more stable than the E-conformer. Recently, only E-C-cyanomethanimine has been detected toward Sagittarius B2(N),19 although both isomers are produced in near equal abundance. In this study the obscurity of the Z-isomer rotational transitions were correlated with its small component dipole moments that yield Received: July 23, 2014 Revised: October 17, 2014
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Figure 1. Numbering of atoms for iminodiacetonitrile (I), the transition state (TS), Z-C-cyanomethanimine (II), ketenimine (III), and E-Ccyanomethanimine (IV) together with their optimized bond lengths (Å) obtained using B3LYP/aug-cc-pvdz level of theory.
temperatures. Therefore, 850 °C was considered the optimum thermolysis temperature. When a chlorobenzene slush bath (−45 °C) trap was used, almost all the unidentified bands vanished. This thermolysis/trapping setup was maintained for 4 h during which time the precursor 8 g was exhausted. Then the contents of the −45 °C trap were warmed up to −22 °C using a carbon tetrachloride slush bath. Immediately, at this temperature, the spectrum of a new species which we have assigned as C-cyanomethanimine, NCCHNH, was detected, but slightly contaminated with unidentified impurities marked with asterisks (*). The K structure of ν6 and ν10 bands of C-cyanomethanimine was resolved using a resolution of 0.086 cm−1.
six times weaker transitions compared to those of the Ecounterpart. In this work we strive to explore, both experimentally and theoretically, the properties of the pyrolysis products of iminodiacetonitrile. An improved Fourier transform infrared (FTIR) spectrum of Z-C-cyanomethanimine will be reported and analyzed. A comprehensive theoretical study of the conformation, vibrational analysis, thermodynamics, and kinetics of these thermolysis products will be launched. The study was also supported by natural bond orbital (NBO) analysis.
2. EXPERIMENTAL DETAILS Solid iminodiacetonitrile, (NCCH2)2NH (I), was purchased from Aldrich and used without further purification. Eight grams of I was placed in a “ROTAFLO” ampule and heated to 180 °C using a heating tape. The “ROTAFLO” ampule was connected to a 25-cm-long and 8-mm-diameter pyrolysis tube. The FTIR spectrum of its vapor at 0.02 Torr was recorded. The pyrolysis tube was heated for ca. 10 cm of its length by a cylindrical electrical furnace. The pyrolysis tube was connected via a trap to a White Cell of ten meters path length. A Bomem DA3.002 F.T.I.R. spectrometer was used. The spectra were recorded at 1.0 and 0.086 cm−1 resolutions and up to 100 scans. A mercury−cadmium−telluride (MCT) detector at liquid nitrogen temperature and a range of 800−6000 cm−1 was used. The precursor, I, was heated at different temperatures (700− 1000 °C) using a Pt/Pt−Rh thermocouple. At all this range of temperatures, large amounts of hydrogen cyanide (HCN) and acetonitrile (CH3CN) have been generated together with the intact precursor. Methane (CH4) has started to show up above 850 °C, while the parent (I) has disappeared completely. New unidentified absorption bands appeared with intensities that are fairly independent of this range of cracking
3. COMPUTATIONAL DETAILS All ab initio molecular orbitals calculations were performed using the Gaussian09 suite of programs.24 The geometries of iminodiacetonitrile, (NCCH2)2NH (I), Z-C-cyanomethanimine, NCCHNH (II), ketenimine, CH2CNH (III), and E-C-cyanomethanimine, NCCHNH (IV), were fully optimized to minima using the B3LYP functional of the density functional theory (DFT), second order Møller−Plesset perturbation theory (MP2) and coupled-cluster with single and double excitations (CCSD) methods with 6-311++G**, aug-cc-pvdz, and aug-cc-pvtz basis sets. The transition state (TS) of I to produce II and III was requested by a TS Berry keyword,25 using B3LYP and MP2 methods with 6-311++G** and aug-cc-pvdz basis sets. The intrinsic reaction coordinates (IRCs)26 were monitored for transition structures that connect the minima in the potential energy surfaces. The resulting imaginary frequencies and IRCs of the displacements of the bond lengths and angles connecting the atoms of interest were envisaged by GaussView27 and Chemcraft28 suites of programs. B
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Table 1. Some Optimized Parameters (Bond Lengths/Å and Bond Angles/Degrees) of Iminodiacetonitrile [(NCH2)2NH] (I), the Transition State (TS), Z-C-cyanoformimine, Z-NCCHNH (II), Ketenimine, CH2CNH (III) Which Were Calculated by Using B3LYP/aug-cc-pvdz Level of Theory parameter
C−H
C−C
CN
CN
NH
aug-cc-pvdz for the basis sets. The analysis of the normal mode of the transition state (TS) imaginary frequency (−562.45 cm−1) showed a large displacement of the N1−C3 and C7−H9 bonds of I, which were eventually dissociated simultaneously and accompanied by the migration of H9 from C7 to N12 (see Figure 1 and Supporting Information Tables S9 and S10) to form II and III. These theoretical findings are in excellent agreement with the our FTIR spectra shown in Figure 3a; that is, the impurities marked
with asterisks could tentatively be assigned to ketenimine (III) and E-C-cyanomethanimine (IV). The presence of IV was later confirmed by Evans et al.17 4.5. Natural Bond Orbital (NBO) Analysis. In Figure 7 are depicted the natural atomic charges of I, TS, II, III, and IV which were estimated by using B3LYP/aug-cc-pvdz level of theory. The positive charge of I is dispersed mainly among the hydrogen atoms. Upon heating the precursor, rotations around C−N σ-bonds took place. This step facilitated a proper orientation for the dissociation of C7−H9 and N1−C3 bonds. G
dx.doi.org/10.1021/jp507397x | J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Table 7. Second Order Perturbation (E(2)) Estimation of the Hyperconjugative Energies (kcal/mol)a of Z-Cyanomethanimine (II), and E-Cyanomethanimine (IV) Which Were Calculated by Using B3LYP/aug-cc-pvdz Level of Theory Interaction σC1−H2→σ*H3−N6 σC1−H2→σ*C4−N5 σC1−H2→σ*C4−N5 σC1−C4→σ*C4−N5 σC1−C4→σ*H3−N6 σC1−N6→σ*C4−N5 σH3−N6→σ*C1−C4 σH3−N6→σ*C1−H2 σC4−N5→σ*C1−C4 a
II 5.11 4.45 4.88 4.20
