Microwave and Quantum Chemical Study of the Hydrazino Group as

Article pubs.acs.org/JPCA

Microwave and Quantum Chemical Study of the Hydrazino Group as Proton Donor in Intramolecular Hydrogen Bonding of (2Fluoroethyl)hydrazine (FCH2CH2NHNH2) Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡ †

Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, Blindern, P.O. Box 1033, 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

Downloaded by UNIV OF MANITOBA on September 11, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.jpca.5b06095

S Supporting Information *

ABSTRACT: The microwave spectrum of (2-fluoroethyl)hydrazine (FCH2CH2NHNH2) was studied in the 11−123 GHz spectral region to investigate the ability of the hydrazino group to form intramolecular hydrogen bonds acting as a proton donor. This group can participate both in five-member and in six-member internal hydrogen bonds with the fluorine atom. The spectra of four conformers were assigned, and the rotational and centrifugal distortion constants of these rotameric forms were determined. Two of these conformers have five-member intramolecular hydrogen bonds, while the two other forms are without this interaction. The internal hydrogen bonds in the two hydrogen-bonded forms are assumed to be mainly electrostatic in origin because the N−H and C−F bonds are nearly parallel and the associated bond moments are antiparallel. This is the first example of a gas-phase study of a hydrazine where the hydrazino functional group acts as a proton donor in weak intramolecular hydrogen bonds. Extensive quantum chemical calculations at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD/ cc-pVQZ levels of theory accompanied and guided the experimental work. These calculations predict the existence of no less than 18 conformers, spanning a CCSD internal energy range of 15.4 kJ/mol. Intramolecular hydrogen bonds are predicted to be present in seven of these conformers. Three of these forms have six-member hydrogen bonds, while four have five-member hydrogen bonds. The three lowest-energy conformers have five-member internal hydrogen bonds. The spectrum of the conformer with the lowest energy was not assigned because it has a very small dipole moment. The CCSD relative energies of the two hydrogen-bonded rotamers whose spectra were assigned are 1.04 and 1.62 kJ/mol, respectively, whereas the relative energies of the two conformers with assigned spectra and no hydrogen bonds have relative energies of 6.46 and 4.89 kJ/mol.

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INTRODUCTION The Oslo lab has for a long time performed microwave (MW) studies of a large number of compounds possessing intramolecular hydrogen (H) bonds. Much of our work has focused on weak proton donor groups. Recently, we reported gas-phase studies of alcohols,1−5 carboxylic acids,6 thiols,7−10 thiolcarboxylic acids,11 selenols,12−15 amines,16−18 amides,19,20 and phosphines,21−24 where the said groups are proton donors in internal H bonds with acceptors such as the fluorine atom,1,6,9,11 the chlorine atom,17,19,23,24 π-electrons of triple bonds,3−5,7,13,15,18 π-electrons of double bonds,2,8,10,14,21 and Walsh25 pseudo-π electrons.12,20,22 References of many of our earlier works are found in the cited literature as well as in reviews.26−29 No gas-phase studies of hydrazines (R−NHNH2), where the hydrazino group (−NHNH2) acts as a proton donor, appear to have been reported. The hydrazino group is a fairly strong proton acceptor and consequently a relatively weak proton donor, just as in the case of its congener, the amino group. In this work, we report the first MW study of (2fluoroethyl)hydrazine, henceforth referred to as FEH. This compound is also the first studied example of a hydrazine that © 2015 American Chemical Society

has conformers stabilized by intramolecular H bonding between H atom(s) of the hydrazino group and the fluorine atom. Two different kinds of internal H bonds may exist for FEH. One of these may form a five-member ring consisting of the F− C−C−N−H chain of atoms, while the other type is composed of the six-member F−C−C−N−N−H ring. Conformers without H bonds are not expected to be very different in energy than rotamers stabilized by this interaction because of the weakness of the H bond interaction. A delicate conformational equilibrium was therefore foreseen to be present in gaseous FEH. The “competition” that might exist between conformers stabilized with five- or six-member H bonds on the one side and rotamers with no H bond on the other was a major motivation for undertaking this research. The orientation of the substituent, R, of a substituted hydrazine R−NHNH2 relative to the NHNH2-group has been used to classify conformers as Inner or Outer,30 and this Received: June 25, 2015 Revised: August 8, 2015 Published: August 10, 2015 9252

DOI: 10.1021/acs.jpca.5b06095 J. Phys. Chem. A 2015, 119, 9252−9261

Article

The Journal of Physical Chemistry A

cooled with small portions of dry ice to ca. −30 °C during the experiments to enhance the intensity of the spectrum. The compound was kept at −80 °C when not in use. Traces of NH3 and N2H4 impurities were observed in the MW spectrum. The spectrum was recorded at a pressure of 5−10 Pa using the Stark-modulation spectrometer of the University of Oslo described in detail elsewhere.33 Two salient features of the spectrometer are the accuracy, which is ∼0.10 MHz for strong and isolated transitions, and the resolution, which is ∼0.5 MHz for such transitions. Measurements were performed in the 11− 123 GHz frequency interval. Radio frequency MW doubleresonance experiments (RFMWDR), similar to those of Wodarczyk and Wilson, 34 were also undertaken. The RFMWDR equipment is described elsewhere.33 This technique makes it possible to assign unambiguously particular transitions.

nomenclature is also followed in the present case. Newman projections of Inner and Outer conformers are shown in Figure 1. Rotational isomerism is possible about the C−C, C−N, and


Figure 1. Newman projections of FCH2CH2NHNH2 viewed along the N−N bond defining the Inner and Outer classification.

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N−N bonds of FCH2CH2NHNH2, which results in a large number of conformers. The MW work is guided by high-level quantum chemical calculations, which indicates that 18 rotameric forms, nine Inner and nine Outer, exist for FEH (see below). These theoretical predictions helped us assign MW spectra of four conformers and allowed unambiguous identification of the rotamers to which the assigned MW spectra belonged.

RESULTS AND DISCUSSION Quantum Chemical Calculations. Frozen-core MP2,35 CCSD,36−39 and B3LYP40,41 calculations were performed employing the Gaussian 0942 and Molpro43 programs running on the Abel cluster in Oslo. The MP2 and B3LYP computations were undertaken using Gaussian 09, whereas the CCSD calculations were performed with Molpro. The default convergence criteria of the two computer programs were used. Dunning’s44 correlation-consistent cc-pVTZ triple-ζ basis set was employed in the MP2 and B3LYP calculations, while the cc-pVQZ basis set, which is of quadruple-ζ quality, was chosen for the CCSD computations. The atom numbering F7−C1(H2,H3)−C4(H5,H6)−N8(H9)−N10(H11,H12) was chosen for FEH. The conformers of this compound can conveniently be described by the values of the F7−C1−C4−N8, C1−C4−N8−N10, C1−C4−N8−H9, C4−N8−N10−H11, and C4−N8−N10−H12 dihedral angles. These angles were varied in a systematic manner to locate potential conformer candidates. The structures of these forms were optimized at the MP2/cc-pVTZ level, and their vibrational frequencies were calculated. A conformer is characterized by having no imaginary frequencies. This was found for 18 rotamers, which were given Roman numerals I− XVIII for reference. The MP2 structures of these forms are listed in Tables S1−S18 in the Supporting Information. Models of the nine Inner rotamers are sketched in Figure 2, while the nine Outer conformers are depicted in Figure 3. The Watson quartic centrifugal distortion constants,45 nuclear quadrupole constants of the two 14N nuclei, and the electronic energies corrected for zero-point vibrational energies were also calculated for the 18 forms, and these parameters are reported in Tables S1−S18. The centrifugal distortion constants were calculated as described by McKean et al.46 It is shown below that eight conformers have relative energies and dipole moments that make their spectra likely candidates for assignments. B3LYP calculations of the vibration−rotation constants (the α’s),47 the sextic centrifugal distortion constants,45 and the differences between the equilibrium rotational constants (re) and the ground-state rotational constants (r0) were performed for these forms. The B3LYP method was chosen because these calculations are much less expensive than using the MP2 method with the same basis set. The MP2 structures were used as starting points in CCSD/ cc-pVQZ structure optimizations. These optimized CCSD structures are assumed to be close to the Born−Oppenheimer equilibrium structures, due to the very high theoretical level of

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EXPERIMENTAL SECTION Synthesis. The synthesis and NMR data of free FEH have never before been reported in detail. FEH was prepared (Scheme 1) starting from 2-fluoroethyl-4-methylbenzenesulfonate synthesized as previously reported.31 Scheme 1. Synthesis of FCH2CH2NHNH2

A modified form of the synthesis of Baklouti and Hedhli32 was used to produce 2-fluoroethylhydrazine: In a 100 mL threenecked flask equipped with a stirring bar, a nitrogen inlet, and a dropping funnel was introduced hydrazine hydrate (30 mL, 0.62 mol). 2-Fluoroethyl-4-methylbenzenesulfonate (19.3 g, 88 mmol) was slowly added for 20 min, while the temperature was maintained at 15 °C with an ice bath. At the end of the addition, the solution was heated to 50 °C and maintained at this temperature for 1 h. After it cooled at room temperature, diethyl ether (20 mL) was added, and the organic phase separated and was discarded (it contained a mixture of the expected product and ∼10% of the disubstituted hydrazine (FCH2CH2)2NNH2). The aqueous phase was then extracted three times with chloroform (50 mL). After the organic phase was dried on magnesium sulfate and filtered, the chloroform was removed in vacuo, and the crude hydrazine was distilled in a vacuum line (0.1 mbar) and selectively trapped in a cell immersed in a bath cooled at −30 °C. Yield: 3.2 g, 47%. 1H NMR (CDCl3, 400 MHz) δ 3.02 (td, 2H, 3JHH = 4.7 Hz, 3JHF = 28.2 Hz, CH2N); 3.19 (s brd, 3H, NHNH2); 4.60 (td, 2H, 3JHH = 4.7 Hz, 2JHF = 47.5 Hz, CH2F). 13C NMR (CDCl3, 100 MHz) δ 55.2 (1JCH = 150.9 Hz (t), 2JCF = 18.9 Hz (d), NCH2); 81.9 (1JCH = 133.5 Hz (t), 1JCF = 164.9 Hz (d), FCH2). Spectroscopic Experiments. FEH is a colorless liquid whose vapor pressure is roughly 150 Pa at room temperature. Fumes of this liquid were admitted to the MW cell, which was 9253


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Figure 2. Inner conformers of (2-fluoroethyl)hydrazine. The numbers in parentheses are the CCSD electronic energies in kJ/mol relative to the energy of conformer XII (Figure 3), which is the global energy minimum conformer. Rotamers I and VI each have a six-member F−C−C−N−N−H internal hydrogen bond, whereas III and V each have an intramolecular five-member F−C−C−N−H hydrogen bond, while II, IV, VII, VIII, and IX all have no hydrogen bond. The MW spectrum of V has been assigned; see text.

Figure 3. Outer conformers of (2-fluoroethyl)hydrazine. Number in parentheses are the CCSD energies in kJ/mol relative to conformer XII. Rotamer XV has a six-member H bond, while XII and XIV have five-member H bonds. The remaining conformers X, XI, XIII, XVI, XVII, and XVIII have no hydrogen bond. The MW spectra of XIII, XIV, and XVI were assigned; see text.

rotational constants, and CCSD principal inertial axes dipole moment components are listed in Table 2. The results in Tables 1 and 2 warrant discussion. The C4− N8−N10−H11 and C4−N8−N10−H12 dihedral angles (Table 1) define the Inner and Outer conformations. It is seen that both dihedral angles vary several degrees among the conformers, between −82.3 and −88.2° for the C4−N8−N10− H11 dihedral angle, while the C4−N8−N10−H12 angle varies

calculations. Vibrational frequencies could not be calculated at this level of theory due to limited resources. The CCSD structures, electronic energies, and dipole moments are listed together with the MP2 results in Tables S1−S18 in the Supporting Information. The characteristic CCSD dihedral angles specifying the conformers are collected in Table 1, while the MP2 energy differences corrected for zero-point vibrational energies, the CCSD electronic energy differences, the CCSD 9254


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The Journal of Physical Chemistry A Table 1. Characteristic CCSD Dihedral Anglesa of Conformers of (2-Fluoroethyl)hydrazineb


dihedral angle

F7−C1−C4−N8

C1−C4−N8−N10

I II III IV V VI VII VIII IX

−70.2 −69.6 −60.2 67.4 65.1 66.9 −176.4 177.4 176.9

79.7 −176.6 −63.4 75.9 −173.2 −68.5 80.7 179.9 −68.1

X XI XII XIII XIV XV XVI XVII XVIII

−94.9 −68.1 −57.1 71.1 65.2 59.6 −177.3 176.0 −179.0

63.4 −179.8 −57.4 74.8 −177.4 −79.1 76.6 173.4 −62.4

C1−C4−N8−H9

C4−N8−N10−H11

Inner conformers −160.6 −57.2 57.5 −163.9 −54.3 53.0 −159.4 −61.5 54.2 Outer conformers −174.8 −58.0 63.8 −163.1 −56.4 43.9 −161.5 −65.2 61.3

C4−N8−N10−H12

−82.3 −86.0 −82.9 −88.4 −83.7 −87.9 −88.2 −85.1 −86.6

32.6 31.0 35.4 29.7 33.1 30.1 29.7 31.6 31.2

80.5 86.8 92.6 88.5 86.4 81.1 86.6 85.0 94.7

−160.7 −154.9 −150.0 −153.1 −156.0 −159.6 −155.2 −157.0 −147.5

In degrees. bAtom numbering: F7−C1(H2,H3)−C4(H5,H6)−N8(H9)−N10(H11,H12). Full CCSD structures are given in Tables S1 − S18 of the Supporting Information. a

Table 2. Theoretical Energy Differences, CCSD Rotational Constants and Principal-Axes Dipole Moment Components of Conformers of (2-Fluoroethyl)hydrazine energy difference (kJ/mol) a

CCSD

2.41 9.39 5.35 7.66 0.73 5.98 6.00 8.10 12.47 14.31 11.79 0.0 6.14 1.62 8.60 4.98 10.73 10.35

3.50 9.67 5.90 8.15 1.04 6.62 5.94 7.76 12.58 15.43 11.98 0.0 6.46 1.63 9.34 4.89 10.33 10.38

MP2 I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII a

b

rotational constants (MHz)

dipole moments (debye)

A

B

C

μa

μb

μc

μtot

8185.0 14462.9 9540.3 10248.3 13657.8 8118.1 14118.0 22287.7 14225.3 8625.4 14477.6 8899.5 10609.3 13771.3 8168.6 14202.2 22917.9 14195.5

3757.0 2420.7 3028.9 2839.8 2551.2 3740.0 2397.1 2053.3 2380.0 3460.4 2432.4 3286.2 2795.2 2542.6 3802.4 2411.3 2046.0 2430.7

2833.9 2267.4 2835.5 2727.8 2335.9 2819.9 2251.0 1953.0 2234.4 2749.0 2265.7 2962.6 2702.1 2329.3 2861.4 2261.9 1950.1 2246.2

0.765 0.416 0.500 0.486 0.071 1.428 0.602 0.632 0.180 2.140 1.539 0.130 2.785 0.574 0.533 3.173 2.116 2.009

2.093 3.014 1.550 1.919 2.579 1.730 0.036 0.469 0.618 1.568 2.188 0.243 1.817 0.007 0.980 0.981 1.258 0.734

1.089 0.257 2.221 2.258 0.907 0.373 0.484 0.332 0.207 0.878 2.045 0.349 0.648 0.476 1.674 0.372 1.703 2.121

2.480 3.054 2.754 3.003 2.735 2.274 0.773 0.854 0.676 2.795 3.367 0.444 3.387 0.745 2.011 3.342 2.993 3.012

MP2/cc-pVTZ corrected for zero-point energies. bCCSD/cc-pVQZ electronic energies.

between 29.7 and 35.5° for the Inner forms. The corresponding variations are somewhat larger for the Outer conformers, between 80.5 and 94.7° for the former dihedral angle, and between −147.5 and −160.7° for the C4−N8−N10−H12 dihedral angle. It is also seen from Table 1 that several of the F7−C1−C4−N8, C1−C4−N8−N10, and C1−C4−N8−H9 dihedral angles differ substantially from the “canonical” values (±60 and 180°). These deviations indicate that nonbonded forces influence the geometries of several of the conformers to a considerable extent. The Outer conformer XII (Figure 2) was found to be the global energy minimum in both the MP2 and in the CCSD

calculations (Table 2), and this rotamer was assigned a relative energy of 0.0 kJ/mol. The MP2 and CCSD energy differences of all 18 conformers are in very good agreement (better than ∼1 kJ/mol), as shown in Table 2. The CCSD energy differences of all 18 conformers span an energy interval of 15.43 kJ/mol (same table). It is impossible to say exactly how accurate the CCSD energy differences are, but these energy differences are likely to be accurate at least to within 2−3 kJ/ mol. The nonbonded distances between the H atom involved in H bonding with the F7 atom should be near or less than 260 pm, which is the sum of the Pauling van der Waals radii48 of H (120 9255


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The Journal of Physical Chemistry A Table 3. CCSD Geometries and Energies of Hydrogen-Bonded Conformers conformer

Na

H bondb

distancec (pm)

angled

anglee(deg)

anglef

angleg (deg)

ΔEh (kJ/mol)

I III V VI XII XIV XV

6 5 5 6 5 5 6

F7···H11 F7···H9 F7···H9 F7···H12 F7···H9 F7···H9 F7···H11

255.2 260.4 248.1 218.9 258.1 247.5 242.9

F7···H11−N10 F7···H9−N8 F7···H9−N8 F7···H12−N10 F7···H9−N8 F7···H9−N8 F7···H11−N10

106.6 97.7 101.8 132.9 96.2 101.4 107.9

C1−F7, H11−N10 C1−F7, N8−H9 C1−F7, N8−H9 C1−F7, H12−N10 C1−F7, N8−H9 C1−F7, N8−H9 C1−F7, H11−N10

54.9 3.7 8.8 50.7 6.5 6.7 46.5

3.50 5.90 1.04 6.62 0.0 1.63 9.34


a The F7 atom can be involved in five- or six-member H bonds; see text. The numbers given in this column indicate how many atoms that are involved in intramolecular H bonding. bAtoms involved in the intramolecular H bond. Dots indicate nonbonded interaction. cNonbonded distance between F7 and the H atom involved in the H bond. dAtoms forming an angle characteristic of the H bond. eAngle in degrees of the characteristic H bond. fAngle between the C1−F7 and the N−H bond involved in H bonding. gAngle in degrees. hElectronic energy relative to conformer XII.

dense with absorption lines occurring every few megahertz throughout the entire spectral range (11−123 GHz) investigated. This was expected for the following reasons: A thermodynamic equilibrium exists for the conformers of FEH, and the concentration of each of them is dictated by Boltzmann’s distribution law. Another factor that results in reduced absolute intensity of the MW spectrum is the fact that each conformer has approximately five normal vibrational modes with frequencies below 500 cm−1 (Tables S1−S18). The population in each vibrational state is determined by Boltzmann statistics. Molecules of each state have their own resolved MW spectrum. It is seen from Table 2 that 10 rotamers (I, III, V, VI, VII, VIII, XII, XIII, XIV, XVI) span a CCSD relative energy interval of less than 8 kJ/mol. The Boltzmann factors of conformers with relative energies larger than 8 kJ/mol are less than 0.02 at −30 °C. Not only concentration, but the intensity of the spectrum is also of great importance for the assignment of a particular conformer. The intensity depends on the square of the principal-axis component of the dipole moment. None of the eight remaining high-energy (>8 kJ/mol) conformers II, IV, IX, X, XI, XV, XVII, and XVIII have very large dipole moments (Table 2) that could outweigh their low concentration, and no searches for their spectra were therefore performed. The relative energies of VII and VIII are 5.94 and 7.76 kJ/ mol (Table 2). Their largest dipole moment components are ∼0.6 D (same table). These two factors and the absolute weakness of the observed spectrum make it very unlikely that it would be possible to assign their spectra, and no efforts were made in this direction. The spectra of the eight remaining lowenergy conformers I, III, V, VI, XII, XIII, XIV, XVI were searched for, and we were able to assign the spectra of four of them, namely, V, XIII, XIV, and XVI, as described below. The following strategy was used to assign the spectra: The B3LYP calculations for the eight conformers in question yielded re- and r0-rotational constants (Tables S1, S3, S5, S6, S12, S13, S14, and S16). The differences (A0 − Ae, etc.) between these B3LYP constants were added to the CCSD rotational constants to obtain the best possible approximation of the experimental r0 rotational constants. These theoretical r0 rotational constants were used together with the MP2 quartic centrifugal distortion constants to predict the approximate frequencies of strong transitions for the conformer under consideration. Searches were then undertaken for these transitions using Stark and RFMWDR spectroscopies. The spectra of conformers with low CCSD relative energies and large dipole moments were first searched for.

pm) and F (140 pm). Inspection of the CCSD structures of the 18 conformers (Tables S1−S18) suggests that seven of them (I, III, V, VI, XII, XIV, and XV) meet this criterion. The nonbonded F···H distances of the seven H-bonded conformers are collected in Table 3 together with several other parameters of relevance for intramolecular H-bonding in these forms. It is seen from this table that three of these conformers, namely, I, VI, and XV, are stabilized by F7−C1−C4−N8−N10−H11 or F7−C1−C4−N8−N10−H12 six-member H bonds, while four (III, V, XII, and XIV) benefit from f ive-member H bonds formed by the F7−C1−C4−N8−H9 link of atoms. The conformers V, XII, and XIV, which all have five-member H bonds, are the three lowest-energy forms of FEH, spanning an energy interval of only 1.63 kJ/mol (Tables 2 and 3), followed by I (3.50 kJ/mol), which has a six-member H bond. The shortest nonbonded H···F7 distance is found in VI, which has a six-member H bond. Interestingly, this conformer is 6.62 kJ/ mol higher in energy than XII, which is stabilized by a fivemember H bond. The conformer stability thus depends not only on the shortness of the H bond. Other nonbonded effects must have a say. A preferred arrangement of many H bonds, especially intermolecular, is the nearly linear arrangement of the, usually three, atoms involved in H bonding. However, the strong directive covalent forces in FEH will not permit the F7···H−N angle to take a value near 180°. It is seen (Table 3) that this angle is far from linear in five-member candidates (96.2− 101.8°). This angle is somewhat larger, ∼107° in the two sixmember conformers, I and XV, where the H11 atom is involved in internal hydrogen bonding, and as large as 132.9° in VI, where H12 takes part in this kind of interaction. Another interesting feature emerges from Table 3: The angles between the C−F and N−H bonds are a few degrees from being parallel in all five-member H-bonded rotamers, while these angles are roughly 50° in the six-member counterparts. The bond moment of the C−F bond is 1.4 D,49 while the bond moment of N−H is 1.3 D.49 The bond moments are nearly antiparallel in the five-member conformers, which is ideal for electrostatic stabilization. It is concluded that this electrostatic interaction is a major reason why the fivemember ring conformers V, XII, and XIV are the lowest-energy forms of FEH. This characteristic H-bond geometry in the fivemember conformers of FEH closely resemble the two corresponding H-bonded conformers of its amino analogue, FCH2CH2NH2,50 as well as the three H-bonded rotamers of a similar amine, namely, F2CHCH2NH2.51 Microwave Spectrum and Assignment Strategy. The observed MW spectrum of FEH is relatively weak and very 9256


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The Journal of Physical Chemistry A Table 4. Spectroscopic Constantsa of Conformer V of FCH2CH2NHNH2 ground Av (MHz) Bv (MHz) Cv (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) HKc (Hz) rmsd Ne

first ex. lowest

second ex. lowest

first ex. second

state

torsion (v30)

torsion (2v30)

lowest torsion (v29)

13517.8137(25) 2521.48377(53) 2309.55877(54) 1.15304(28) −11.6853(34) 90.9152(62) −0.217667(28) −0.010336(13) 0.000339(42) 0.0193(10) −0.4973(30) 1.795(20) 1.067 611

13417.1942(71) 2521.3270(17) 2310.6401(17) 1.1963(16) −11.9247(59) 79.03(34) −0.226588(71) −0.007486(30)

13321.338(14) 2522.0262(67) 2312.1600(66) 1.228(13) −12.0168(95) 69.12(66) −0.23607(11) −0.005104(37)

13667.2030(93) 2513.1865(55) 2303.3259(54) 1.0887(89) −11.9942(65) 108.36(44) −0.207489(57) −0.012650(26)

1.441 151

1.466 92

1.225 124

theoryb 13 637.0 2512.0 2304.4 1.18 −11.7 87.3 −0.219 −0.0101


a

S-reduction Ir-representation.45 Uncertainties represent one standard deviation. bThe theoretical r0-rotational constants have been obtained from the CCSD and B3LYP calculations; see text. The centrifugal distortion constants are from the MP2 calculations; see text. cFurther sextic centrifugal distortion constants preset at zero. dRoot-mean-square deviation defined as rms2 = ∑[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number of spectroscopic constants used in the fit. eNumber of transitions used in the fit. The spectra are listed in Tables S19−S22 of the Supporting Information.

HKJ, and HK, were needed in the fit to get a root-mean-square deviation comparable to the experimental uncertainties. Attempts to include further sextic constants were made, but these constants had large uncertainties; no really improved fit was obtained in this way. These sextic constants were ultimately preset at zero. It is interesting to compare the experimental findings with the theoretical predictions. Comparison of the experimental rotational constants with the theoretical rotational constants (Table 2) shows that the spectrum might belong either to V or to XIV. However, the dipole moment components (Table 2) exclude XIV. The assignment of the spectrum of Table S19 to V is thus considered to be secure. Comparison of experiment and theory is in order: The experimental ground-state rotational constants (Table 4, second column) and the theoretical counterparts obtained from CCSD and B3LYP calculations are in good agreement in the cases of B and C, while that of A deviates by 0.88%. It is assumed that most of the deviation in the case of A is most likely due to the B3LYP calculations of the third derivatives of the potential energy hypersurface at the equilibrium structure. This method is hardly sufficiently refined to yield highly accurate values for the zero-point vibrational contribution to the rotational constants. Table 4 also reports the MP2 values of the quartic centrifugal distortion constants, which are seen to agree very well with the experimental equivalents. The same cannot be said about the B3LYP sextic centrifugal distortion constants (Table S5), which deviate so much from the experimental values in Table 4 that a discussion is meaningless, and they have therefore not been included in this table. The reason for this failure is again assumed to be difficulties in obtaining accurate third derivatives. Vibrationally Excited State of V. The spectrum of the ground vibrational state of V was accompanied by several satellite spectra that were assumed to belong to vibrationally excited states of this rotamer. Three excited-state spectra consisting of 151, 82, and 124 transitions listed in Tables S20 − S22, respectively, were assigned. Their spectroscopic constants

Assignment of the Spectrum of V. This H-bonded conformer has the second-lowest relative energy, which is 1.04 kJ/mol higher than the energy of XII (Table 2). Conformer V has a small μa = 0.07 D and a large μb = 2.58 D, while μc = 0.90 D (Table 2). Searches were first made for b-type Q-branch transitions, which were predicted to be the strongest transitions of the spectrum of V. The theoretical r0-spectroscopic constants in the last column of Table 4 were used to predict the approximate frequencies of these transitions, which were readily found. These bQ-transitions are also among the strongest transitions of the entire spectrum. bR-branch transitions were searched for next and found with ease. The assignments were gradually extended to include higher and higher values of the principal J quantum number up to Jmax = 88. Searches for transitions involving even higher values of J were performed, but no such lines could be unambiguously assigned because they are too weak due to an unfavorable Boltzmann factor. Finally, many c-type lines were assigned. They have generally much lower intensities than the b-type transitions, because μc is less than μb. The frequencies of μc lines could be very accurately predicted using the spectroscopic constants obtained from the b-type transitions. The frequencies of hypothetical a-type lines could now be calculated with very high precision, but these lines were not found because they are too weak caused by a very small μa = 0.07 D (Table 2). No effects due to quadrupole coupling of the two 14N nuclei with the molecular rotation were observed in the spectrum of this rotamer as well as in the spectra of the three other conformers assigned in this work. Sørensen’s least-squares fitting program Rotfit52 was used in the assignment procedure above. A total of 611 transitions (Table S19 of the Supporting Information) could be fitted to within their experimental uncertainties using Watson’s Sreduction Ir-representation Hamiltonian.45 Centrifugal distortion is prominent for the high J-transitions, with a maximum of more than 4 GHz (Table S19). The experimental spectroscopic constants are shown in Table 4, second column. Accurate rotational and quartic centrifugal distortion constants were obtained. Four sextic centrifugal distortion constants, HJ, HJK, 9257


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The Journal of Physical Chemistry A are listed in Table 4, columns 3−5. No accurate sextic centrifugal distortion constants were obtained from the leastsquares fits in these cases, and only quartic centrifugal distortion constants were used in the final least-squares fits. The two first spectra are assumed to belong to the first and second excited states of the lowest torsional vibration (v30), whereas the last spectrum is assumed to be the first excited state of the second-lowest torsional vibration (v29). The MP2 calculations predict frequencies of 116 and 150 cm−1 for v30 and v29, respectively, compared to 105(20) and 137(25) cm−1 obtained from rough relative intensity measurements. The experimental vibration−rotation constants (the α’s)47 were computed by subtraction of the rotational constant of the excited state under consideration from the corresponding ground-state rotational constant. In this manner, αA = 100.6195(75), αB = 0.1568(18), and αC = −0.7682(18) were found for the first excited state of v30 from the entries in Table 4. The B3LYP values are 98.47, − 1.24, and −1.54 MHz (Table S5), which is considered to be good agreement given the approximate nature of the theoretical calculations. The experimental values of the corresponding rotation−vibration constants of v29 are −149.3893(96), 8.2973(55), and 6.2329(55) MHz. The B3LYP equivalents are −148.63, 8.94, and 6.56 MHz in good agreement with experiment. Assignment of the Spectrum of XIII. The relative CCSD energy of this form, which is without a H bond, is 6.47 kJ/mol, and its Boltzmann factor is only 0.04 relative to XII. Its major dipole moment component is μa = 2.79 D (Table 2). This conformer is nearly a symmetrical top with Ray’s asymmetry parameter53 κ = −0.976. The approximate frequencies of its aRspectrum were predicted using the theoretical ground-state rotational constants (Table 5, last column) obtained as described above, and the MP2 centrifugal distortion constants. This spectrum has characteristic pile-ups associated with a-type R-branch transition. Figure 4 shows the J = 14 ← 13 pile-up region observed for this conformer.

Figure 4. J = 15 ← 14 pile-up region of conformer XIII taken at a Stark field of ∼150 V/cm. The numbers above the peaks indicate the value of the K−1 pseudo quantum numbers of the transitions. The K−1 = 8 and 11 transitions are overlapped by other transitions.

The first unambiguous assignment of this very weak spectrum was obtained for aR lines using the RFMWDR technique. Recording of the spectrum at relatively low Stark fields eliminates transitions with slow Stark effects, while aRtransitions of XIII with K−1 > 2 are completely modulated at low fields. The much-simplified spectrum obtained in this manner was very helpful for the assignments of many additional a R-transitions. This rotamer has a sizable μb = 1.82 D (Table 2). Searches for b-type transitions were undertaken, but none were unambiguously assigned, presumably due to low intensities as well as the crowded nature of the spectrum with many overlapping absorption lines. The spectroscopic constants obtained from 181 aR-transitions (Table S23) are listed in Table 5. The DK and d2 centrifugal distortion constants could not be determined from the fit and were therefore preset at zero. One sextic centrifugal distortion constant, HKJ, was needed to get a satisfactory fit. Comparison of the experimental rotational constants (Table 5) with the theoretical rotational constants (Table 2) reveals that conformer XIII is the only suitable candidate. The assignment of the spectrum is Table S23 conformer is thus considered to be unambiguous. The differences between the experimental and theoretical rotational constants are 1.54, 0.35, and 0.17%, for A0, B0, and C0, respectively, which is considered to be satisfactory given the approximations involved in the theoretical calculations. The experimental and theoretical quartic centrifugal distortion constants (Table 5) are in good agreement in the cases of DJ and DJK, while a large difference is seen for d1. Assignment of the Spectrum of XIV. The energy of this conformer, which has an internal five-member H bond, is only 1.63 kJ/mol higher than the energy of the global-minimum form XII. The largest CCSD dipole moment component of XIV, μa, is as low as 0.57 D, while μb is almost zero, and μc is only 0.48 D (Table 2). Ray’s asymmetry parameter, κ, is −0.963. A very weak a-type R-branch spectrum with features very similar to that of conformer XIII (previous paragraph) should exist for this rotamer as well. RFMWDR searches were made for the spectrum of XIV using the theoretical spectroscopic constants (Table 6) to predict suitable candidates

Table 5. Spectroscopic Constantsa of the Ground Vibrational State of Conformer XIII of FCH2CH2NHNH2 A0 (MHz) B0 (MHz) C0 (MHz) DJ (kHz) DJKc (kHz) d1 (kHz) HKJd (Hz) rmse Nf

experiment

theoryb

10745.5(57) 2772.657(15) 2677.566(14) 2.9764(19) −29.128(24) −0.267(19) −0.933(95) 1.222 181

10 579.8 2762.9 2672.8 2.78 −26.2 −0.488

a

S-reduction Ir-representation.45 Uncertainties represent one standard deviation. bThe theoretical r0-rotational constants have been obtained from the CCSD and B3LYP calculations; see text. The centrifugal distortion constants are from the MP2 calculations; see text. cDK and d2 kept at zero in the fit; see text. dFurther sextic centrifugal distortion constants preset at zero. eRoot-mean-square deviation defined as rms2 = ∑[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number of spectroscopic constants used in the fit. f Number of transitions used in the fit. The spectrum is listed in Table S23 of the Supporting Information. 9258


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The Journal of Physical Chemistry A Table 6. Spectroscopic Constantsa of the Ground Vibrational State of Conformer XIV of FCH2CH2NHNH2 experiment A0 (MHz) B0 (MHz) C0 (MHz) DJ (kHz) DJKc (kHz) d1 (kHz) HKJd (Hz) rmse Nf

13723(11) 2514.552(55) 2303.728(56) 1.1535(14) −12.656(11) −0.238(13) −0.588(33) 0.982 201

Table 7. Spectroscopic Constantsa of Conformer XVI of FCH2CH2NHNH2

theoryb Av (MHz) Bv (MHz) Cv (MHz) DJ (kHz) DJK (kHz)d d1 (kHz) HKJe (Hz) rmsf Ng

13781.5 2507.8 2300.8 1.18 −12.7 −0.217


a−f Comments as for Table 5. The spectrum is listed in Table S24 of the Supporting Information.

ground state

lowest torsion (v30)

theoryb

14 017.1(22) 2391.4824(69) 2240.4157(71) 0.826 32(99) −11.0917(88) −0.0906(80) −0.248(23) 1.101 187

c

14 167.5 2386.2 2238.2 0.795 −11.7 −0.117

13 927 2399.265(29) 2244.651(32) 0.8676(25) −11.010(13) 0.0c 0.0c 1.658 82

a−b Comments as for Table 5. cFixed. d−gComments as for c−f Table 5. The spectra are listed in Tables S25 and S26 of the Supporting Information

for this kind of spectroscopy. These transitions were soon identified, and the pile-ups of this conformer became evident when the spectrum was recorded at relatively low Stark fields. Ultimately, 201 transitions (Table S24) were assigned and used to determine the spectroscopic constants displayed in Table 6. Searches for c-type lines were unsuccessful, presumably because of the small μc, as well as the complexity of the spectrum. The rotational constants of XIV (Table 6) are similar to those of V (Table 4), but the assignment of the spectrum of Table S24 to the latter rotamer is assumed to be secure, because no a-type spectrum was observed for V, while an a-type spectrum is the only kind of spectrum observed for XIV. The experimental and theoretical rotational constants agree to within 0.43, 0.26, and 0.01% in the cases of A0, B0, and C0, which is good agreement. The same can be said about the quartic centrifugal distortion constants. Assignment of the Spectrum of XVI. This conformer, which is without a H bond, has a CCSD energy that is 4.89 kJ/ mol higher than the energy of XII (Table 2). The Boltzmann factor of this form is thus 0.09 at −30 °C. XVI has a large μa = 3.17 D and κ = −0.974 resulting in a characteristic aR-pile-up spectrum similar to those described in the two previous cases. The spectrum is comparatively weak, but it is stronger than the spectra of XIII and XIV. It was in fact so strong that it was readily observed in the survey spectra. The spectrum of XVI was assigned in the same manner as described for the spectra of these two conformers. μb is 0.98 D (Table 2), but attempts to find the strongest b-type lines failed presumably because of the low concentration of XIV. Attempts to find c-type lines also failed. This was expected because μc = 0.37 D (Table 2). Ultimately, 185 aR-transitions with Jmax = 26 and K−1max = 20 listed in Table S25 of the Supporting Information were assigned and used to determine the spectroscopic constants listed in Table 7. The centrifugal distortion constants DK and d2 could not be obtained for similar reasons as described above for XIII and XIV. The rotational constants in Table 7 are similar to the rotational constants of II, VII, and IX. Rotamer II is excluded because μa is as small as 0.42 D and its CCSD relative energy is as high as 9.67 kJ/mol. VII and IX are also excluded for similar reasons. The assignment of the spectrum in Table S25 to conformer XVI is therefore considered to be unambiguous. The theoretical r0 rotational constants shown in the last column of Table 7 are in good agreement with their experimental counterparts in column 2. The largest difference is seen for A0, where the theoretical constant is 1.1% larger than the experimental equivalent. There is quite good agreement

between the experimental quartic centrifugal distortion constants DJ, DJK, and d1 and the MP2 results (Table 7). Vibrationally Excited State of XVI. A weak spectrum with approximately half of the intensity of the ground-state spectrum of XVI and with similar Stark modulation properties and RFMWDR behavior was observed close to the ground-state spectrum of XVI. This spectrum, which is listed in Table S26 and whose spectroscopic constants are displayed in Table 7, is assigned as the first excited torsional state spectrum of the lowest torsional vibration (v30) of XVI. This spectrum was so weak that no unambiguous assignment could be made for K−1 lines less than 3. The A rotational constants will therefore be very uncertain. This constant was fixed at 13 927 MHz, which was derived by subtracting the vibration−rotation constant, αA = 90.44 MHz obtained in the B3LYP calculations (Table S16), from the A0 constant (Table 7). The MP2 vibrational frequency is 100 cm−1 for this mode, while the B3LYP values of αB = −5.32 and αC = −2.76 MHz (Table S16). The equivalent experimental values obtained from the entries in Table 7 are αB = −7.88(12) and αC = −4.13(12) MHz. The Unsuccessful Searches. We searched for the spectra of eight candidates, namely, I, III, V, VI, XII, XIII, XIV, and XVI. The spectra of V, XIII, XIV, and XVI were assigned, while no assignments were obtained for I, III, VI, and XII. The relative CCSD energy of conformer I is 3.50 kJ/mol, and its major dipole moment component is μb = 2.09 D (Table 2). Extensive searches have been made for its b-type Stark spectrum, and the RFMWDR technique was also employed in searches for its a-type spectrum, but to no avail. It is possible that the energy difference is larger than 3.50 kJ/mol and that this is the reason why success was not achieved. Candidate III has a relative energy of 5.90 kJ/mol (Boltzmann factor = 0.05 at −30 °C), and this may perhaps explain why it was not found in extensive searches for its c-type spectrum. Futile RFMWDR experiments were also performed. Similar experiments were made for VI, whose CCSD energy is 6.62 kJ/mol higher than the energy of XII (Table 2) with the same conclusion The global energy minimum conformer XII has μa = 0.13, μb = 0.24, and μc = 0.35 D, according to the CCSD calculations (Table 2). The MW spectrum of this form will consequently be extremely weak and would be almost impossible to assign in a crowded Stark spectrum, such as the one at hand. Nevertheless, futile attempts to assign the spectrum of XII were made, using both Stark and RFMWDR spectroscopies. 9259




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CONCLUSIONS MP2/cc-pVTZ and CCSD/cc-pVQZ calculations indicate that there are 18 different conformers of FCH2CH2NHNH2. Seven of these conformers are stabilized by intramolecular hydrogen bonds, where the hydrazino group acts as proton donor and the fluorine atom is proton acceptor. Three of these rotamers have six-member hydrogen bonds, whereas four have five-member hydrogen bonds. The three lowest-energy conformers are all stabilized by five-member hydrogen bonds demonstrating that the hydrazino group indeed may act as a proton donor in hydrogen bonds. The reason why the majority of the conformers with fivemember H bonds are more stable than the rotamers with sixmember internal H bonds is presumed to be a favorable orientation of the C−F and N−H bonds involved in H bonding. These two bonds are nearly parallel, and their bond moments are nearly antiparallel in the five-member species, which is ideal for electrostatic interaction. A similar favorable situation does not exist in the six-member forms. The energies of the 18 rotamers span a CCSD energy interval of 15.4 kJ/mol. The spectra of eight of the 18 conformers were assumed to be possible candidates for assignment. The reasons for excluding 10 conformers were a combination of high relative energies and/or small dipole moment components. The absolute intensity of the MW spectrum is small, as expected. Nevertheless, spectra belonging to four different rotamers, two with five-member hydrogen bonds and two without H bonds, were assigned, and rotational and centrifugal distortion constants were determined. Searches for the spectra of the remaining four conformers were performed. The spectrum of the conformer with the lowest energy of all conformers was not assigned, because its dipole moment is very small. Two conformers have relative energies larger than ∼6 kJ/mol, and this energy difference is assumed to be the major reason why their spectra were not found. The fourth conformer, whose spectrum was not assigned despite a relative energy of 3.5 kJ/mol and a high dipole moment, is more difficult to assess, but an underestimate of the energy difference by the MP2 and CCSD methods could be the reason.

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ACKNOWLEDGMENTS This work was supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/ V30). It also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K). J.-C.G. thanks the Centre National d’Etudes Spatiales (CNES) for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06095. Results of the theoretical calculations, including MP2/ccpVTZ and CCSD/cc-pVQZ electronic energies, molecular structures, rotational constants, and dipole moments. MP2 harmonic vibrational frequencies, rotational and quartic centrifugal distortion constants. B3LYP/cc-pVTZ vibration−rotational constants, differences between r0 and re rotational constants, and sextic centrifugal distortion constants. Microwave spectra of four conformers. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald. [email protected]. Notes

The authors declare no competing financial interest. 9260


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Microwave and Quantum Chemical Study of the Hydrazino Group as

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