C2v Form Research Articles

On the detectability of interstellar diaminomethane ((NH2)2CH2)

ABSTRACT The recent synthesis of diaminomethane ((NH2)2CH2) under simulated interstellar conditions implies that this molecule is likely present in the interstellar medium (ISM), and this work provides the highly accurate quantum chemically computed rotational constants and IR frequencies for comparison to experiment that may aid in its potential future detection. This simplest geminal diamine of prebiotic note has a transition for its ν18 fundamental frequency at 702.5 cm−1 (14.23 μm) with an intensity on level with that of the antisymmetric stretch in carbon dioxide. Additionally, its 1.72 D dipole moment for the most stable C2v conformer should make it detectable with radiotelescopes as well. The C2 conformer may also be detectable in similar regions where the C2v form is present, but only warmer or higher-energy regions will allow for possible observation of the C1 conformer. In any case, rotational constants for these other two conformers are also provided in addition to the full set of fundamental vibrational frequencies for the C2v conformer. These data will assist with further laboratory classification and possible interstellar detection of this prebiotic molecule.

Exploring the structural and conformational properties of dioxygen dihalides (halogen = F, Cl, Br)

The structural and conformational properties of dioxygen difluoride (1), dioxygen dichloride (2), and dioxygen dibromide (3) have been investigated by means of the hybrid density functional theory (B3LYP) and the hybrid meta exchange-correlation functional (M06-2X) with the aug-cc-pVQZ, aug-cc-pVTZ, cc-pVTZ, and 6-311++G** basis sets and natural bond orbital interpretation. The results obtained showed that the rotation at the O–O bond by passing from the plane symmetrical trans-(C2h) (or cis-(C2v)) form leads to the O–F bonds breaking. The natural bond order analysis revealed that the O–O bonds in the C2h and C2v forms of compound 1 possess double bond characters and the small bond orders between the oxygen and fluorine atoms results from the strong electron delocalization between the lone pairs of the fluorine atoms (LP4F) and the antibonding orbitals of the adjacent O–O double bond (π*O–O). The lengthening and shortening of the O–F and O–O bonds, respectively, in the trans-(C2h) or cis-(C2v) forms of compound 1 can be interpreted by the decrease of the bonding interactions between the fluorine and oxygen orbitals. The profiles of the orbital amplitudes (or electron densities) of O–O and O–F bonds in the C2h and C2v forms of compound 1 revealed that there are strong electronic repulsions between the πO–O and the lone pairs of oxygen atoms with the lone pairs of fluorine atoms, leading to the dissociation of O–F bonds. It is worth noting that there are strong hyperconjugative interactions between LP2O and the antibonding orbitals of adjacent O–F bonds (σ*O–F) in the skew (C2) form of compound 1, leading to the decrease of the O–O bond length and the increase of the F–O bond length by increasing the πO-O bonding and the σ*O–F antibonding orbital occupations. The increase of the electron delocalization from LP3F to σ*O–O with the decrease of the σ*O–O–σO–O energy gap (results from the increase of the O–O bond length) justifies the similarity between the adiabatic O–O bond dissociation energies in compound 1 and HOOH. The electron delocaizations from LP2O2 to σ*O3–X (X = F (1), Cl (2), Br (3)) decrease drastically from the skew ground state (C2) forms of compound 1 to compound 2 but increase slightly from compound 2 to compound 3, causing the significant increase of the O–O bond length ongoing from compound 1 to compound 2 and the slight decrease from compound 2 to compound 3. Effectively, the conformational properties of compounds 1–3 can be interpreted with the principle of maximum softness.

Exploring Water Binding Motifs to an Excess Electron via X2–(H2O) [X = O, F

X(2)(-)(H(2)O) [X = O, F] is utilized to explore water binding motifs to an excess electron via ab initio calculations at the MP4(SDQ)/aug-cc-pVDZ + diffs(2s2p,2s2p) level of theory. X(2)(-)(H(2)O) can be regarded as a water molecule that binds to an excess electron, the distribution of which is gauged by X(2). By varying the interatomic distance of X(2), r(X1-X2), the distribution of the excess electron is altered, and the water binding motifs to the excess electron is then examined. Depending on r(X1-X2), both binding motifs of C(s) and C(2v) forms are found with a critical distance of ∼1.37 Å and ∼1.71 Å for O(2)(-)(H(2)O) and F(2)(-)(H(2)O), respectively. The energetic and geometrical features of O(2)(-)(H(2)O) and F(2)(-)(H(2)O) are compared. In addition, various electronic properties of X(2)(-)(H(2)O) are examined. For both O(2)(-)(H(2)O) and F(2)(-)(H(2)O), the C(s) binding motif appears to prevail at a compact distribution of the excess electron. However, when the electron is diffuse, characterized by the radius of gyration in the direction of the X(2) bond axis with a threshold of ∼0.84 Å, the C(2v) binding motif is formed.

ChemInform Abstract: A Computational Evaluation of Relative Stabilities of the Open C2v and Cyclic D3h Forms of Ozone

Abstract In addition to the open (C2v) form, the ozone molecule can also exist in the higher-lying cyclic (D3h) form. In the present study four different computational evaluations of the relative energy difference between the two forms (12,19,50 and 122 kJ mol−1) were employed to study the influence of temperature on the two conformers. It is shown that the results obtained strongly depend on the energy difference used. The threshold temperature at which the cyclic D3h, structure reaches the 1% mole fraction level in the equilibrium mixture with the C2v form varies from about 350 to about 3800 K. Similarly, although the highest value of the contribution of isomerism to the heat capacity δCp,1 for the equilibrium mixture is always about 1.8 J K−1 mol−1, the temperature at which the maximum is reached varies from about 650 to about 6600 K. The possible role of the cyclic isomer is discussed, including its relation to the dissociation limit of ozone.

Electronic Transitions of CsC2, CsC2-, and CsC4 in Neon Matrixes

The anions of CsC2 and CsC4 produced by sputtering a graphite surface with Cs+ were mass-selected and trapped in neon matrixes at 6 K. The electronic absorption spectra of CsC2 and CsC4, obtained by photodetachment of electrons from the anions, were measured subsequently and reveal strong absorptions in the visible range, which resemble the known band systems of C2- and C4-, respectively. The origin band of CsC2 (500.4 nm) and CsC4 (442.6 nm) is shifted by approximately 1100 or by approximately 700 cm(-1) to the blue from the position of the 0(0)0 band of C2- or C4-. The observed system of CsC2 is assigned to the 2B2-X 2A1 electronic transition of the T-shaped form. The CsC4 spectrum is consistent with a C4- chain slightly perturbed by the Cs atom. The oscillator strength of the observed electronic transition of CsC2 and CsC4 is an order of magnitude larger than for the respective carbon anions. CsC2- has a weak electronic transition, assigned to 1B2-X 1A1 in the C2v form, with origin band at 516.5 nm.

The Oxygen-Rich Carboxide Series: COn (n = 3, 4, 5, 6, 7, or 8)

The lowest-energy isomers of the high-order carboxide series, COn (n = 3-8), have been elucidated via ab initio quantum chemical calculations. The structures of the lowest order of these (3 and 4) are in close agreement with previous calculations and with experimental data. The structures of the higher-order species are elucidated and correlated to the previous structures, showing similarities in structure and reactivity pathways. The reaction energies of the formation of all products are shown to be related. Exothermic pathways of formation often involve a C2v form of CO2, which was shown to be metastable. The newly identified species could be intermediates in atmospheric reactions. The calculated vibrational frequencies and IR intensities may be used to identify these metastable species.

Conformational stabilities of dicyclopropyl ketone determined from variable temperature infrared spectra of xenon solutions and ab initio calculationsElectronic supplementary information (ESI) available: Calculated energies, diagonal force constants, symmetry coordinates and calculated asymmetric torsional potential function. See http://www.rsc.org/suppdata/cp/b4/b400142g/

The infrared (3500–40 cm−1) spectra of gaseous and solid dicyclopropyl ketone, (c-C3H5)2CO, along with the Raman (3500–40 cm−1) spectra of the liquid and solid phases have been recorded. Only the major cis–cis (C2v) conformer (CO bond over the two three-membered rings) and minor cis–trans (Cs) form (CO bond eclipsing the hydrogen atom on one of the rings) have been identified in the fluid phases. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. Utilizing six different combinations of pairs of bands from the C2v and Cs conformers, the average enthalpy difference between these two conformers has been determined to be 530 ± 27 cm−1 (6.34 ± 0.32 kJ/mol), with the C2v form the more stable rotamer. At ambient temperature it is estimated that there is only 13 ± 2% of the Cs conformer present. A complete vibrational assignment is given for the C2v form and several of the fundamentals of the Cs conformer have been identified by utilizing predictions of fundamental frequencies, infrared intensities, and Raman activities from MP2(full)/6-31G(d) ab initio calculations. The structural parameters, dipole moments and conformational stabilities have been obtained from ab initio calculations at the level of restricted Hartree–Fock (RHF), with full electron correlation by the perturbation method to second order (MP2(full)) and hybrid density functional theory (DFT) by the B3LYP method with a variety of basis sets. The predicted conformational stabilities are consistent with the experimental results. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Potential Energy Surfaces for LiH2 and Photochemical Reactions Li*+ H2 ↔ LiH + H

Potential energy surfaces for the Li*+ H2 → LiH + H and the reverse reactions are calculated using ab initio methods. Extensive configuration interactions have been done for a large number of collinear, C2v, and Cs geometrical forms using a large basis set to describe the 2s−3d atomic states of the lithium atom and the neutral and anionic hydrogen molecule. The Li(3s) channel has a small activation barrier and leads to a stable intermediate, 4 2A‘ (LiH2)*, but a diabatic coupling between the 3 and 4 2A‘ surfaces would preferentially lead to a nonreactive inelastic collision producing Li(2p). The Li(2p) channel leads to one entirely repulsive potential surface (3 2A‘) and two attractive potential surfaces (2 2A‘ and 1 2A‘ ‘) resulting in two stable intermediates. The Li(2p) atoms with enough collision energy to overcome the endothermicity of 1624 cm-1 can lead to the reaction producing LiH. Those three excited intermediate complexes are bent (planar) and not linear. The charge transfer from the metal atom to the hydrogen molecule does not occur in the course of the collision but much later, when one hydrogen atom breaks away, implying that the harpooning model does not apply to this case. The reverse reaction LiH + H → Li + H2 can produce (i) the high-energy Li(2s) directly, (ii) the thermal Li(2p), and (iii) the thermal Li(2s) accompanied with a chemiluminescence. The 2 2A‘ intermediate may play an important role in the reverse reaction, too.

An extreme problem revisited: The hyperpolarizability of the open and cyclic forms of ozone

Single and double excitation coupled-cluster [CCSD(T)] theory including an estimate of connected triple excitations by a perturbational treatment, calculations with a specially optimized [7s5p4d2 f] basis set, consisting of 168 contracted Gaussian-type functions, yield mean values β̄=−14.0 e3a03Eh−2 and γ̄=2642 e4a04Eh−3 for the first and second dipole hyperpolarizability of the open (C2v) form of ozone. Due to the higher predictive capability of the CCSD(T) method and the improved completeness of the basis set, we expect these values to be more accurate than those obtained with the CCD+ST(CCD) method and a smaller [6s4p3d1f] basis set [G. Maroulis, J. Chem. Phys. 101, 4949 (1994)]. The hyperpolarizability of the cyclic (D3h) form of ozone, calculated with a [7s5p4d2 f] basis set optimized for the ring structure, is β=−6.5 e3a03Eh−2 and γ̄=1549 e4a04Eh−3. In both symmetries, electron correlation changes drastically the self-consistent field picture of the hyperpolarizability tensor. Fourth-order Møller–Plesset perturbation theory is seen to predict very reliable values for the hyperpolarizability of the cyclic form. This is in marked contrast to the state of things brought forth by the study of the open form.

Photochemistry of (OCS)n− cluster ions

We report the photochemistry of (OCS)n− cluster ions following 395 nm (n=2–28) and 790 nm (n=2–4) excitation. In marked contrast to (CO2)n−, extensive bond breaking and rearrangement is observed. Three types of ionic products are identified: S2−(OCS)k, S−(OCS)k/OCS2−(OCS)k−1, and (OCS)k−. For n<16, 395 nm dissociation is dominated by S2−-based fragments, supporting the theoretical prediction of a cluster core with a C2v (OCS)2− dimer structure and covalent C–C and S–S bonds. A shift in the branching ratio in favor of S−-based products is observed near n=16, consistent with an opening of the photodissociation pathway of OCS− core-based clusters. These monomer-based cluster ions may coexist with the dimer-based clusters over a range of n, but electron detachment completely dominates photodissociation as long as their vertical electron detachment energy, increasing with addition of each solvent molecule, is less then the photon energy. An (OCS)2− conformer of C2 symmetry with a covalent C–C bond is believed to be responsible for 790 nm dissociation of (OCS)2−, yielding primarily OCS− products. The yield of OCS−, and thus the importance of the C2 form of (OCS)2− cluster core, decreases with increasing n, perhaps due to more favorable solvation of the C2v form of (OCS)2− and/or a solvent-induced increase in the rate of interconversion of conformers. The (OCS)k− products observed in 395 nm photodissociation of the larger (n⩾7) clusters are attributed to photofragment caging. Formation and dissociation mechanisms of clusters with different core types are discussed. The photochemical properties of (OCS)n− are compared to those of the isovalent (CO2)n− and (CS2)n− species.

Carbonyl Hypofluorite-A Density Functional Theoretical Study

Calculations on FOC(O)OF are reported at the B3LYP/6-311+G(2df)//B3LYP/6-31+G(d)+ZPC level of theory. The molecule is predicted to exist as a mixture of rotamers. The lowest-energy rotamer, the syn−anti, is predicted to be planar. The syn−syn rotamer appears to be conformationally floppy; a nonplanar (C2) form, with O−F bonds rotated from the CO3 plane by 16.4°, and a planar (C2v) form are separated by only 0.01 kcal/mol. The anti−anti rotamer is predicted to be nonplanar (C2 symmetry); the O−F bonds are rotated out of the CO3 plane by 27.9°. Calculated barriers for interconversion of rotamers are less than 10 kcal/mol. We also calculate vibrational frequencies for the rotamers. The calculated O−F bond enthalpy is 30.2 kcal/mol.

Quantum mechanical frequencies and matrix assignments to Al2H2

The three lowest lying isomers of Al2H2, the planar dibridged hydrogen D2h form, the planar vinylidene-like C2v form, and the planar monobridged hydrogen Cs form, were examined using high level ab initio methods. The highest level treatment of the energetics and harmonic vibrational frequencies, TZ2Pf CCSD(T), suggests that the dibridged and monobridged forms of Al2H2 were detected by matrix isolation infrared studies (J. Phys. Chem. 97, 10 295 1993).

Theoreticalab InitioStudy of CN2O2Structures: Prediction of Nitryl Cyanide as a High-Energy Molecule

Structures, energies, and harmonic vibrational frequencies of CN2O2 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules and in a study of the mechanism of the reaction between NCO and NO radicals. Nitrosoisocyanate, ONNCO (1), earlier studied as a collision complex in the reaction of NCO and NO (Lin, M. C.; Melius, C. F. J. Phys. Chem. 1993, 97, 9124) is the most energetically favorable CN2O2 isomer, but its 18 kcal/mol unimolecular dissociation barrier is very low. Thus 1 can only be observed as a short-lived intermediate. However, nitrosofulminate, ONCNO (8), and nitryl cyanide, NCNO2 (12), higher energy isomers (69 and 38 kcal/mol above trans-1a, respectively), are more stable than 1 toward decomposition. This offers species 8 and 12 as interesting molecules for experimental study. Moreover, 12 can be a reasonably stable molecule as its C−N bond dissociation energy (59 kcal/mol) and the barrier to decomposition into N2 and CO2 (54 kcal/mol) are rather high, being comparable to those of nitromethane. The estimated large values of the heat of formation (ΔHf°300 = 60 kcal/mol) and of the decomposition energy of 12 (12 → N2 + CO2; ΔE = 150 kcal/mol) make this species potentially interesting as a high-energy molecule. Our study also includes four- (2) and three-membered (17) cyclic and bicyclic (3) isomers. The Cs cyclic isomers, 2 and 17, are extremely unstable, but the bicyclic C2v form (3) has a 29 kcal/mol dissociation barrier and should be observable.

Interconversion of diborane (4) isomers

Highly correlated electronic structure calculations using many-body perturbation theory (MBPT) and coupled-cluster (CC) gradient techniques are applied to a study of a reaction pathway which links the two forms [C2v and D2d ] of diborane(4) [B2H4]. A reaction coordinate which preserves C2 symmetry is studied, as this mechanism is allowed by orbital symmetry rules. However, calculations show that the minimum energy path does not conform to this idealized mechanism. Rather, the reaction coordinate bifurcates, and the transition state contains no nontrivial elements of symmetry. At the level of partial fourth-order many-body perturbation theory [SDQ-MBPT(4)] with a large triple-zeta plus double polarization basis set, differences in distances between the hydrogen atoms corresponding to the bridge atoms in the C2v form and the two boron atoms [δr(B–H)=‖r(B1–H)−r(B2–H)—] are 0.14 and 0.81 Å, reflecting the pronounced asymmetry of the transition state structure. We find that the C2v isomer should be the thermodynamically favored form of B2H4. At the coupled-cluster singles and doubles level with a noniterative treatment of triple excitation effects, the barrier to isomerization is found to be 6.3 kcal/mole using a large generally contracted basis set. This result, along with statistical arguments concerning the rate of reaction, suggest that equilibrium may be established relatively rapidly, and that rigid molecule treatments of B2H4 spectra may not be appropriate.

A computational evaluation of relative stabilities of the open C2v and cyclic D3h forms of ozone

In addition to the open ( C 2v) form, the ozone molecule can also exist in the higher-lying cyclic ( D 3h) form. In the present study four different computational evaluations of the relative energy difference between the two forms (12,19,50 and 122 kJ mol −1) were employed to study the influence of temperature on the two conformers. It is shown that the results obtained strongly depend on the energy difference used. The threshold temperature at which the cyclic D 3h, structure reaches the 1% mole fraction level in the equilibrium mixture with the C 2v form varies from about 350 to about 3800 K. Similarly, although the highest value of the contribution of isomerism to the heat capacity δ C p,1 for the equilibrium mixture is always about 1.8 J K −1 mol −1, the temperature at which the maximum is reached varies from about 650 to about 6600 K. The possible role of the cyclic isomer is discussed, including its relation to the dissociation limit of ozone.

Electron spin resonance study on the structure and dynamics of propane cation radical: The intrinsic form with a C S symmetry

Propane cation radicals with CS symmetry were found by electron spin resonance in C3F8 and C2F6 at 4 K. They have one C–C bond elongated considerably, and the unpaired electron is in the 10a′ orbital of CS symmetry, which is derived from the 4b1 orbital of C2V symmetry by the deformation. The highest spin density appears in the elongated C–C bond, and the in-plane methyl proton at the other end has the largest hyperfine coupling (hfc) of about 9 mT. The degree of deformation from the C2V form is dependent on the matrices. From the general consideration on the structure of propane cation, it is deduced that the cation in SF6, which has been considered to have C2V symmetry, also has CS symmetry with a smaller distortion, and that distortion to CS symmetry is an intrinsic nature of the propane cation.

Peroxy and cyclic isomers of NO2 and NO−2

Ab initio molecular electronic structure methods have been used, in conjunction with large basis sets, to investigate the possible existence of isomers of the conventional C2v open-chain NO2 and its anion, NO−2. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for both the ‘‘peroxy’’ and cyclic isomers of these species at the self-consistent-field (SCF), single and double excitation configuration interaction (CISD), coupled cluster including all single and double substitutions (CCSD), and complete active space (CAS) SCF levels of theory utilizing both double-zeta plus polarization (DZ+P) and quadruple-zeta plus double polarization (QZ+2P) basis sets. For comparison, ‘‘normal’’ (C2v open-chain) NO−2 was also studied. Diffuse functions were added to these basis sets to provide a better description of the electron densities of the anionic isomers. The four peroxy and cyclic structures investigated were predicted to be relatively high-lying (≥60 kcal mol−1) minima at most levels of theory considered. Moreover, interconversion between the C2v and Cs open-chain forms appears unlikely both for NO2 and NO−2.

Valence ionization and the electron affinities of the open and cyclic forms of Se3 and Te3

Se3 and Te3 exist as the S3 molecule in an open C2v and a closed D3h form with different electronic configurations but nearly the same total energy. The ionization spectra of Se3 and Te3 are calculated in both the C2v and D3h forms by many-body Green’s function methods. The D3h forms behave quite regularly, but very strong correlation effects are found in the C2v forms. If a theory accurate to third order is used, satellite lines appear at the ionization threshold. If higher order terms are included the satellite lines at the ionization threshold experience a slight change in energy, but a considerable reduction of intensity. Se3 has a positive electron affinity in both the C2v and D3h forms (2B1, 2A′2, states), and Te3 appears to have two positive electron affinities (2B1, 2A1 for the C2v form and 2A2 and 2E′ for the D3h form).

Theoretical studies of the singlet and triplet potential energy surfaces of cyclobutanediyl

AbstractCyclobutanediyl (2) has been studied in both its singlet and triplet states by ab initio electronic structure theory. The triplet, which is the ground state of the molecule, exists in both C2h and C2v forms which interconvert via a Cs transition state. For the singlet, only a C2h form is found. It passes, via a Cs transition state, onto the C2v surface on which bicyclobutane (3) is the only minimum. The ring‐flipping (inversion) process in 3 includes the singlet biradical as an intermediate, and involves a novel, non‐least motion path similar to one previously proposed by Gassman. Semiclassical periodic orbit theory indicates that the various minima on both the singlet and triplet surfaces can interconvert via quantum mechanical tunneling.

Ab initio Studies on Hydrazines: 1H-Pyrrol-1-amine (N-Aminopyrrole)

The geometries of four stationary structures of 1H-pyrrol-1-amine have been optimized with the 3-21G and 3-21G(N*) basis sets. The lowest- energy and only equilibrium structure is the 'perpendicular' CS form (4) in which a pyramidal NH2 group is bisected by the plane of the pyrrole ring. The transition structure for inversion at the NH2 group is the perpendicular C2V form (2). After zero-point vibrational -energy corrections, it lies 24.5 kJ mol-1 [3-21G(N*)] above (4). The transition structure for rotation about the N-NH2 bond is the 'parallel' CS form (3) in which a plane of symmetry bisects both the pyrrole ring and the attached pyramidal NH2 group; it lies 26.5 kJ mol-1 above (4). The planar C2V structure (1) is a second-order saddle point lying 69.4 kJ mol-1 above (4). The spacing of NH-stretching frequencies calculated for the equilibrium structure (4) of 1H-pyrrol-1-amine is in the range for normal primary amines, unlike that for 1,1-dimethylhydrazine.