Microwave and Quantum Chemical Study of Intramolecular Hydrogen

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Article

Microwave and Quantum Chemical Study of Intramolecular Hydrogen Bonding in 2-Propenylhydrazine (HC=CHCHNHNH) 2

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Harald Møllendal, Svein Samdal, and Jean-Claude Guillemin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11141 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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The Journal of Physical Chemistry

Microwave and Quantum Chemical Study of Intramolecular Hydrogen

Bonding

in

2-Propenylhydrazine

(H2C=CHCH2NHNH2)

Harald Møllendal,*,ϯ Svein Samdal,ϯ and Jean-Claude Guillemin‡

ϯCentre

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

Received: November 13, 2015 Revised: December , 2015

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ABSTRACT: The microwave spectrum of 2-propenylhydrazine (H2C=CHCH2NHNH2) was studied in the 12 – 61 and 72 – 123 GHz spectral regions. A variety of intramolecular hydrogen bonds between one or more of the hydrogen atoms of the hydrazino group and the π-electrons are possible for this compound. Assignments of the spectra of four conformers, all of which are stabilized with intramolecular hydrogen bonds are reported. One hydrogen bond exists in two of these conformers, whereas the π-electrons are shared by two hydrogen atoms in the two other rotamers. Vibrationally excited states spectra were assigned for three of the four conformers. The internal hydrogen bonds are weak, probably in the 3 – 6 kJ/mol range. A total of about 4400 transitions were assigned for these four forms. The microwave work was guided by quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. These calculations indicated that as many as 18 conformers may exist for 2-propenylhydrazine and 11 of these have either one or two intramolecular hydrogen bonds. The four conformers detected in this work are among the rotamers with the lowest CCSD electronic energies. The CCSD method predicts rotational constants that are very close to the experimental rotational constants. The B3LYP calculations yielded quartic centrifugal distortion constants that deviated considerably from their experimental counterparts in most cases. The calculation of vibration-rotation constants and sextic centrifugal distortion constants by the B3LYP method were generally found to be in poor agreement with the corresponding experimental constants.

Key words: conformer, rotamer, rotational constants, quartic centrifugal distortion constants, sextic centrifugal distortion constants, vibration-rotation constants, dipole moment, quantum chemical calculations, B3LYP, CCSD


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INTRODUCTION In a recent communication,1 we reported the microwave (MW) spectrum of (2fluoroethyl)hydrazine, FCH2CH2NHNH2. Our reason for investigating this compound was to see whether the hydrazino group would participate in intramolecular hydrogen (H) bonding, acting as a weak proton donor, with the highly electronegative fluorine atom being the acceptor. MW spectra of four conformers of FCH2CH2NHNH2 were assigned and two of these rotamers are indeed stabilized by internal H bonding. The electronegative fluorine atom is a unique and relatively strong acceptor in H bonds and the question is whether the hydrazino group would be capable of forming H bonds as a proton donor with other atoms or groups than the exceptional fluorine atom. It is well established that the π-electrons of double bonds may act as a proton acceptor. Studies of 2-propenylic (allylic) compounds have shown that moderately strong intramolecular H bonding between the πelectrons and the hydroxyl group exists in H2C=CHCH2OH.2-5 It is more surprising that one or more conformers of each of H2C=CHCH2NH2,6-10 H2C=CHCH2SH,11 and H2C=CHCH2SeH12 are stabilized by this interaction since the amino, thiol and selenol groups are significantly weaker proton donors than the hydroxyl group. A study of 2-propenylhydrazine (H2C=CHCH2NHNH2) should reveal whether the hydrazino group is capable of forming internal H bonds with πelectrons. However, an important difference exists between 2-propenylhydrazine and the four other compounds just referred to. In the title compound, the N‒H and NH2 moieties of the hydrazino group may both form internal H bonds, whereas only one proton is available for H bonding in H2C=CHCH2X, where X = OH, NH2, SH, and SeH. The many interesting aspects of H bonding in 2-propenylhydrazine motivated us to carry out the first MW study of this compound, henceforth referred to as 2PH. The present study is a continuation of our extensive 3 ACS Paragon Plus Environment


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studies of intramolecular H bonding. References for much of our work in this field are found in Ref. 1. 2PH has complex conformational properties because rotation about the HC‒CH2, CH2‒ NH, and NH‒NH2 single bonds may result in a large number of conformers. Substituted hydrazines, RNHNH2, occur as what is traditionally referred to as Inner or Outer conformers13 depending on the orientation of the substituent, R, with regard to the NH2 group. In Inner rotamers, the substituent R and the lone electron pair of the NH2 group are antiperiplanar, whereas R and the lone pair are synclinal in Outer conformers. An illustration of Inner and Outer forms are shown in Fig. 1. 2PH also contains the allyl group (H2C=CHCH2‒). Allylic compounds, H2C=CHCH2X, where X is a substituent, generally prefer C=C‒CH2‒X synperiplanar conformers where the C=C‒C‒X dihedral angle is close to 0º, or anticlinal forms where this dihedral angle is about 120º. In 2PH, the conformational properties associated with the allyl group come in addition to the Inner/Outer conformational properties of the hydrazino group. The quantum chemical calculations discussed below indicate that 18 conformers should be taken into consideration. These rotamers are depicted in Fig. 2 (9 Inner forms) and in Fig. 3 (9 Outer conformers). Each form is given a Roman numeral (I – XVIII) for reference. An analysis below indicates that as many as 11 of these forms are stabilized by internal H bonds where either the ‒NH or the ‒NH2 part are proton donor. MW spectroscopy is ideal for studies of complex conformational equilibria such as the one presented by 2PH due to its unparalleled accuracy and resolution. High-level quantum chemical methods are now capable of predicting rotational constants, energy differences between


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conformers as well as dipole moment components along principal inertial axes that are so accurate that the assignment of crowded MW spectra is greatly facilitated. A combination of MW spectroscopy and quantum chemical methods was therefore chosen for this investigation.

EXPERIMENTAL SECTION Synthesis. The reported synthesis of 2PH14 was modified to obtain a pure compound. Hydrazine hydrate (50 mL, 1 mol., 6 equiv.) was introduced in a two-necked flask under nitrogen. The reagent was cooled to 15 °C and allyl bromide (20.2 g, 0.167 mol, 1 equiv.) was added dropwise under stirring while the temperature was maintained below 30°C. At the end of the addition, the mixture was heated to 70 °C and maintained at this temperature for one hour. After cooling to room temperature, diethyl ether (30 mL) was added and the organic phase was separated and discarded. The same procedure was applied with a second addition of diethyl ether (30 mL). Both organic fractions contained a mixture of mono- and diallylhydrazine. Chloroform (50 mL) was then added to the aqueous phase and the organic phase was isolated. After removing the solvent, monoallylhydrazine was distilled in vacuo (0.1 mbar) and selectively condensed in a trap cooled to ‒40 °C. A pure product (> 97%) was thus obtained in a modest 27 % yield (3.25 g, 45 mmol). Unambiguous 1H and 13C NMR data of mono- and diallylhydrazine are given below. This information regarding the NMR spectra was not achieved in the previous work.142Propenylhydrazine: 1H NMR (CDCl3, 400 MHz) δ 3.05 (s brd, 3H, NHNH2); 3.35 (d, 2H, 3JHH = 6.4 Hz, CH2N); 5.20 (d, 1H, 3JHHcis = 13.2 Hz, C=CH(H)); 5.21 (d, 1H, 3JHHtrans = 17.8 Hz, C=CH(H); 5.78 (ddt, 1H, 3JHHtrans = 17.8 Hz, 3JHHcis = 13.2 Hz, 3JHH = 6.4 Hz, C=CH). 13C NMR (CDCl3, 100 MHz) δ 58.3 (t, 1JCH = 134.2 Hz, CH2N); 118.6 (d, 1JCH = 158.4 Hz, CH2); 134.5 (d,



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JCH = 151.9 Hz, CH). 1,1-Di-2-propenylhydrazine: 1H NMR (CDCl3, 400 MHz) δ 2.85 (s brd,

2H, NH2); 3.13 (d, 4H, 3JHH = 6.4 Hz, CH2N); 5.15 (d, 2H, 3JHHcis = 13.2 Hz, C=CH(H)); 5.21 (d, 2H, 3JHHtrans = 17.8 Hz, C=CH(H); 5.85 (ddt, 2H, 3JHHtrans = 17.8 Hz, 3JHHcis = 13.2 Hz, 3JHH = 6.4 Hz, C=CH). 13C NMR (CDCl3, 100 MHz) δ 63.6 (t, CH2N); 118.3 (d, CH2); 134.4 (d, CH).) Spectroscopic Experiments. 2PH is a colorless liquid at room temperature and has a vapor pressure of roughly 160 Pa. Fumes of this liquid were admitted to the MW cell that was cooled with small quantities of dry ice to a temperature of approximately ‒40 ºC in order to enhance intensity of the spectrum as much as possible. Lower temperatures, which would have been desirable because of increased spectral intensities, could not be reached due to insufficient vapor pressure of 2PH. The MW spectrum was recorded in the 12.4 – 61 and 72 ‒ 123 GHz spectral regions. The 50 kHz Stark modulated spectrometer used in this study has been described in details elsewhere.15 Quantum Chemical Calculations. B3LYP16,17 and CCSD18-21 computations were performed employing the Gaussian 0922 program package. Dunning’s23 correlation-consistent ccpVTZ triple-ζ basis set was used in all calculations. The frozen-core approximation was utilized. The default convergence criteria of Gaussian 09 were observed. The atom numbering chosen for 2PH is C1(H6,H7)‒C2(H8)‒C3(H9,H10)‒N4(H11)‒N5(H12,H13). Figures with atom numbering of each of the 18 conformers are shown in the Supporting Information together with their full structurers. It is convenient to describe the conformers of this compound by the values of the C1‒C2‒ C3‒N4, C2‒C3‒N4‒N5, C2‒C3‒N4‒H11, C3‒N4‒N5‒H12, and C3‒N4‒N5‒H13 dihedral angles. A systematic variation of these angles was used to locate potential conformer candidates. 6 ACS Paragon Plus Environment

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The structures of these candidates were optimized at the B3LYP/cc-pVTZ level. Harmonic and anharmonic vibrational frequencies, nuclear quadrupole coupling constants of the two 14N nuclei, Watson's quartic and sextic centrifugal distortion constants24 (observing McKean's precautions25), vibration-rotational constants (the α's)26, equilibrium r(e) and effective (r0) rotational constants were calculated using the optimized B3LYP structures of the 18 conformers shown in Figures 2 and 3. No imaginary harmonic vibrational frequencies were found for these 18 forms and this was taken as evidence that a minimum on the potential energy hypersurface (a conformer) had been identified. Selected B3LYP results of the 18 rotamers are listed in Tables S1 – S18 of the Supporting Information. The B3LYP structures were used as starting points in CCSD/cc-pVTZ calculations of optimized structures and dipole moments as well as its components along the principal inertial axes. The CCSD structures should be close to the Born-Oppenheimer equilibrium structures, due to the very high theoretical level of calculations. Computation of CCSD harmonic vibrational frequencies were not performed because these calculations are costly. The full CCSD structures are listed in Tables S1 – S18 in the Supporting Information together with the B3LYP results. The CCSD values of the five characteristic CCSD dihedral angles are repeated in Table 1. Rotamer I was found to have the lowest energy both in the B3LYP and in the CCSD calculations. The B3LYP energies were corrected for harmonic zero-point vibrational energy contributions. Their energies relative to the energy of I are listed in Table 2. The relative CCSD electronic energies, rotational constants, principal-axes components of the dipole moments, as well as the total dipole moments of the 18 forms are displayed in the same table.



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Comments are warranted for the results presented in these two tables. The C3‒N4‒N5‒ H12 and C3‒N4‒N5‒H13 dihedral angles (Table 1) define the Inner and Outer conformations. Both these angles vary several degrees among the conformers. In one series of Inner forms (conformers I – VI) the C3‒N4‒N5‒H12 dihedral angle varies between 81.7 and 88.1º, whereas the C3‒N4‒N5‒H13 angle takes values between ‒28.2 and ‒33.3º. In the other series of Inner conformers (rotamers VII – IX), the former dihedral angle varies from ‒84.4 to ‒87.2º, and the latter angle has a minimum value of 29.2º, and a maximum value of 31.2º. There are, of course, six different rotamers of the second series to which VII – IX belong, but three of them are mirror images of conformers belonging to the first series (I – VI). These enantiomers have identical spectroscopic properties. Similar variations are encountered for the Outer forms. It is also seen from Table 1 that several of the C1‒C2‒C3‒N4, C2‒C3‒N4‒N5, and C2‒ C3‒N4‒H11 dihedral angles deviate in some cases by as much as ≈ 15º from their "canonical" values (0, ±60, ±120, and 180º). These deviations from normality can presumably be explained largely by the influence of non-bonded forces on the geometries. Table 2 reveals that the trends of the relative B3LYP and CCSD energies are similar. The largest deviation is about 1.8 kJ/mol (conformer VI) between the relative B3LYP and CCSD energies. The CCSD energies of all 18 conformers span a rather narrow interval of 10.28 kJ/mol. It is not possible to say how accurate the CCSD relative energies are, but it is assumed that they are at least accurate to within 2 – 3 kJ/mol. Hydrogen Bonding. The fact that both the ‒NH‒ and ‒NH2 parts of the hydrazino group may be engaged in internal H bonding requires that at least one of the H11, H12, H13 atoms is close to the π-electrons of the C1C2 double bond. Inspection of the interatomic distances of the


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CCSD structures in Tables S1 – S18 of the Supporting Information shows that this occurs for as many as 11 of the 18 rotamers. One H bond is found in most cases, but there are also examples where the π-electrons are involved in two H bonds at the same time. The two H atoms in questions share the π-electrons in these cases. H bond characteristics of these 11 rotamers are listed in Table 3. The criteria for selecting these conformers as candidates for intramolecular H bonding have been that at least one of the nonbonded distances between the H11, H12, and/or H13 atoms and the C1 and/or C2 carbon atoms is less than 290 pm. This distance is the sum of the Pauling van der Waals radii27 of the H atom (120 pm) and the half-thickness of an aromatic molecule (170 pm). In the last IUPAC definition28 of the H bond, it is stated that nonbonded distances between the H atom and the acceptor is not alone a good measure of H bonding. However, we have used the 290 pm limit just for practical purposes. H bonds are sometimes seen as a result of ring closure. 4, 5, or 6 atoms are often involved in these rings. 4-member rings include C2, C3, N4, and H11 in our case. The 5-member rings are composed of C2, C3, N4, N5, and H12, or H13, whereas the 6-member rings consist of C1, C2, C3, N4, N5, and H12, or H13. Table 3 shows that all these different types of internal H bonding are present in one or more of the conformers of 2PH. Two H bonds are present in VI, VII, XI, and XIV. Two H atoms share the π-electrons in each of these cases. One H bond is present in each of the remaining seven conformers. Most interestingly, the relative CCSD energies of the H bonded conformers (Table 3) demonstrate that there is no simple relation between the non-bonded distance(s) and the relative energies. Conformer I, which represents the global energy minimum, has only one internal H 9 ACS Paragon Plus Environment


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bond characterized by a nonbonded H12 ••• C2 distance of 287.9 pm. This bond is only 2.1 pm less than 290 pm, which was used above as a practical criterion for internal H bonding. Rotamer I is, for example, 2.49 kJ/mol lower in energy than VI, which has two H bonds. The nonbonded distance, 250.3 pm, between H13 and C2 in VII (Table 3) is the shortest such contact that exists in the conformers of 2PH. Moreover, this rotamer has two H bonds. VII is nevertheless 0.33 kJ/mol higher in energy than I. The conclusion is that there is no simple relationship between the conformer energy and the shortness or number of H bonds. The majority of the H-bonded conformers spans an energy range of about 5 kJ/mol, whereas conformers without H bonds span an energy interval of 7 – 10 kJ/mol relative to I (Table 2). The fact that the hydrazino group form different H bonds makes it difficult on this basis to roughly estimate the strengths of the H bonds in each case, but 3 – 6 kJ/mol depending on the type of interaction is a plausible estimate given these data. Microwave Spectrum and Assignment Strategy. 2PH has, as expected, a relatively weak and extremely dense MW spectrum. This is due to several reasons. A thermodynamic equilibrium exists in our experiments and each quantum state is populated according to Boltzmann statistics. The 18 conformers span a relatively narrow energy interval of about 10 kJ/mol and 8 rotamers have CCSD energies that differ by less than 5 kJ/mol (Table 2). Each conformer has 6 normal modes with vibrational frequencies less than 500 cm‒1 (Tables S1 – S18 of the Supporting Information). Each conformer will have its own resolved spectrum not only for the ground vibrational state but for vibrationally excited states as well. Obviously, all these factors lead to a low Boltzmann population even for the ground vibrational state of the most


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abundant conformer. The same reasoning explains why the observed spectrum is comparatively weak and extraordinary dense and that transitions of various spectra frequently overlap. The nine conformers II, III, IX, X, XIII, XIV, XVI, XVII, and XVIII have CCSD energies relative to I that are 7 kJ/mol or more higher (Table 2). Their Boltzmann factors relative to I are hence less than 0.03 at ‒40 ºC. No attempts to assign the spectra of any of these conformers were made due to their presumed low abundance. Searches have been performed for the remaining nine forms using the CCSD rotational constants and the B3LYP quartic centrifugal distortion constants to predict their approximate spectra. The Stark modulation patterns and radio frequency microwave double resonance29 (RFMWDR) experiments were used extensively in order to assign the spectra. The least-squares fits of the spectra were done using Watson's Hamiltonian in the S-reduction form with the Ir-representation24 employing Sørensen's program Rotfit.30 No resolved 14N nuclear quadrupole coupling was observed. The ground vibrational state spectra were accompanied by two particularly strong spectra of vibrationally excited states of the same conformer. These two spectra belong to the first excited states of the torsions about the C2‒C3 bond (v33) and the C3‒N4 bond (v32). The B3LYP frequencies of these torsions are in the 80 – 160 cm‒1 region (Tables S1 –S18) and their Boltzmann factors are consequently in the 0.6 – 0.4 interval. Once the spectrum of the ground vibrational state of a rotamer had been assigned, searches were made for the spectra of the excited states of these two torsional modes of this rotamer. The B3LYP rotation-vibration constants (the α’s, Tables S1 – S18) were added to the rotational constants of the ground vibrational state in order to get the best possible prediction of the spectrum of each of the two excited states.



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The third lowest vibrational state is the lowest bending vibration (v31), whose B3LYP frequencies is roughly 250 cm‒1 (Tables S1 – S18) and the Boltzmann factor is about 0.2. The first excited-state spectra of this mode were only searched for when the ground-state spectrum was comparatively strong. No assignments of spectra belonging to this excited state could be made. Assignment of the Ground Vibrational State Spectrum of Conformer I. This globalenergy minimum rotamer has its largest dipole moment components along the a- and c-principal inertial axes, whereas µb is predicted to be small (0.216 D). Survey spectra revealed a relatively strong spectrum close to that predicted using the theoretical spectroscopic constants shown in the last column of Table 4. The first unambiguous assignments of transitions of this form were achieved for a-type R-branch transitions using the RFMWDR method. A typical example of a spectrum obtained by this method is shown in Fig. 4. These assignments were readily extended to include additional a-type R-branch transitions up to J = 21 and K‒1 = 17. The preliminary rotational and quartic centrifugal distortion constants obtained from the aR-spectrum were next used to predict the frequencies of strong cQ-transitions, which were found close to the predicted frequencies. Additional c-type Q- and R-branch lines were gradually included the least squares fit. Ultimately, cQ-lines with Jmax = 60 and cR-lines with Jmax = 52 were assigned. The frequencies of b-type lines should be very precisely predicted from this spectrum. However, no definite assignments of this kind of transitions could be made, presumably because of a small µb (about 0.2 D; Table 2) producing insufficient intensities of the lines in question. The full spectrum consisting of 600 transitions are listed in Table S19 and the spectroscopic constants are repeated in the first column of Table 4.


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Comparison of the experimental r0-rotational constants in column 1 of Table 4 with the CCSD constants in the last column reveals that the CCSD constants are larger than the r0rotational constants by 0.55, 0.44, and 0.52% in the cases of A, B, and C, respectively, which is very satisfactory. The fact that CCSD rotational constants are a little larger than the effective rotational constants are expected in most cases because the CCSD structures are close to the Born-Oppenheimer equilibrium structures. The CCSD bond lengths are generally smaller than the r0-bond lengths. The consequence of this is that the CCSD principal moments of inertia are slightly smaller and the rotational constants are larger than the r0-rotational constants, just as observed in the present case. The B3LYP calculations yielded the r0- and re-rotational constants (Table S1). However, differences between these two sets of rotational constants, Ae – A0, etc, are very different from their counterparts obtained from the experimental r0- and rCCSD-rotational constants. Similar results were obtained for the other three conformers below. The B3LYP method is obviously not capable of producing reliable values for these parameters in the present case. This is unfortunate because accurate predictions of these differences would have help tremendously for the assignment of these complicated spectra. All experimental quartic centrifugal distortion constants, but DK, are larger than the B3LYP equivalents. The largest deviation is observed for d1 (14.4%) and the smallest for DK (0.7%). Much larger differences are seen for the sextic centrifugal distortion constants, which is natural given the simplifications involved in the B3LYP calculations. Inspection of Table 2 shows that the CCSD rotational constants of I differ so much from the other rotational constants that it can be concluded that the spectrum shown in Table S19



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undoubtedly belong to rotamer I. The observation of a- and c-type spectra corroborates this conclusion. Vibrationally Excited States of I. Five spectra belonging to vibrationally excited states of I were assigned. These spectra are listed in Tables S20 – S24 of the Supporting Information and the spectroscopic constants are collected in Table 4. The assignments of several aR-lines of each excited state were confirmed by RFMWDR experiments. It was possible to assign c-type lines up to Jmax = 46 for the strongest vibrationally excited state. The B3LYP calculations (Table S1) predict that this is the first excited state of the torsion about the C2‒C3 bond (v33). Accurate values were obtained for all quartic centrifugal distortion constants of this excited state as shown in the second column of Table 4. Three sextic constants were also obtained. The remaining four sextic constants were kept constant at the ground-state values in the least squares fit. Only aR-branch transitions were identified for the four other excited states. The maximum value of J was typically about 20 and K‒1max approximately 16 in these cases. It was not possible to get accurate values for the quartic constant DK of these states and the value of the ground vibrational state was used and kept constant in the fits of these spectra. One sextic constant, HKJ, was also fitted keeping the remaining sextic constants fixed at zero. The calculated values of the vibration-rotation α-constants of the excited states are listed at the bottom of Table 4 and used to assign the corresponding spectra to individual excited states, as described below. The torsion about the C2‒C3 bond (v33) is the lowest vibrational frequency according to the B3LYP calculations, which predict 91.6 cm‒1 for this vibration (Table S1). Relative intensity measurements yielded 111(20) cm‒1. The B3LYP values of the vibration-rotation constants of the 14 ACS Paragon Plus Environment

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first excited state of the torsion about the C2‒C3 bond (v33) are αA = ‒33.11, αB = ‒9.75, and αc = ‒2.64 MHz (Table S1) compared to the experimental values ‒47.3182(55), ‒5.5640(12), and ‒ 0.1540(12) MHz, respectively (Table 4, column 3). There is only an order of magnitude agreement between theory and experiment in this case. Obviously, B3LYP calculations are not sufficiently refined to produce accurate values in this case as well. The spectroscopic constants listed in the third column of Table 4 are assumed to belong to the second excited state of the C2‒C3 torsion. The α-constants of this state are not exactly twice as large as the constants of the first excited state, which would have been the case for a completely harmonic vibration. However, their changes go in the expected direction and have plausible magnitudes. It is therefore concluded that v33 is a very anharmonic vibration. An alternative assignment of this spectrum as belonging to the first excited state of the lowest bending vibration (v31) is ruled out because α = 10.55 MHz (Table S1), whereas ‒7.85 MHz is Β

the experimental value (Table 4) The constants of column 4 of Table 4 are assumed to belong to the first excited state of the torsion about the C3‒N4 bond (v32). Relative intensity measurements yielded 137(20) cm‒1 for this vibration compared to the B3LYP harmonic frequency of 149.7 cm‒1. The B3LYP method predicts αA = ‒72.12, αB = 13.17, and αC = 8.21 MHz (Table S1). The absolute values of these constants are all larger than their experimental counterparts (Table 4), but they have correct signs and relative magnitudes. Column 5 contains the spectroscopic constants of the second excited state of v32. The αvalues of this state are almost exactly twice as large as the α -values of the first excited state of



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this mode. The torsion about the C3‒N4 bond, v32, thus appears to be essentially harmonic and therefore behaves quite differently from the torsion about C2‒C3 bond (v33) discussed above. The constants of a combination mode of the two torsional vibrations are given in the sixth column. This assignment has been made because the α-constants associated with this excited state are almost equal to the sum of the corresponding constants of the first excited states of v33 and v32. Assignment of the Spectrum of the Ground Vibrational State of VI. The electronic CCSD energy of this conformer is 2.49 kJ/mol higher than the energy of I (Table 2). The Boltzmann factor of VI is 0.28 relative to I at the temperature in question (‒40 °C). The CCSD method predicts that this conformer has µa = 1.417, µb= 0.566, and µc = 1.003 D (Table 3). The a

R-spectrum was assigned in the same manner as described for the assignment of the

corresponding spectrum of I. Ultimately, 171 aR-lines with Jmax = 18 and K‒1max = 16 were assigned as shown in Table S25 of the Supporting Information. Many K‒1-pairs of cQ-branch lines with K‒1 = 6 to K‒1 = 12 coalesce and have rapid Stark effects and this was used to assign several members of them. A few cR-branch lines were also assigned. b-type transitions were not found, presumably because of their weakness. This is in accord with a small µb (see above). Attempts to find spectra of vibrationally excited states of this conformer were made, but no further unambiguous assignments could be made due to the weakness of the spectrum. The CCSD rotational constants are all larger than the experimental rotational constants by 0.55, 0.99, and 0.75 % in the cases of A, B, C, respectively, as expected (see above). All B3LYP quartic centrifugal distortion constants have absolute values that are larger than the experimental


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equivalents. Deviations between 17% (DJ) and 42 % (DJK) were found in this case, which is not satisfactory. Inspection of Table 2 reveals that there are no other conformers predicted to have rotational constants and a major µa similar to VI. The assignment of the spectrum of Table S25 to VI is unequivocal. Assignment of the Spectrum of the Ground Vibrational State of VII. This conformer is 0.33 kJ/mol less stable than I according to the CCSD method. µa is much larger (1.814 D) than the other two dipole moment components, which are 0.020 and 0.229 D, respectively (Table 2). The assignments of the spectra of the ground vibrational state and three vibrationally excited states were made in a manner analogous to those described for I and VI above. Searches for btype and c-type transitions resulted in no assignment, which is due to the small values of the corresponding dipole moment components. The spectrum of the ground state consisting exclusively of 311 aR-transitions with Jmax = 21 and K‒1max = 19 is listed in Table S26 and the spectroscopic constants including quartic and one sextic constant are shown in Table 6. The CCSD rotational constants are larger than the experimental counterparts in the case of A by 1.15 %, and C by 0.25 %, whereas the CCSD value of B is smaller than the effective equivalent by 0.22 %. The B3LYP quartic centrifugal distortion constants are all too large. The rotational constants of VII are similar to the rotational constants of XI. However, the major dipole moment component of VII is along the a-inertial axis, whereas XI has its major component along the c-axis. This is sufficient to conclude that the spectrum of Table S26 undoubtedly belongs to conformer VII.



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Vibrationally Excited States of VII. The aR-spectra (Tables S27 – S29) of three such states were assigned. The two lowest B3LYP vibrational frequencies are the torsions about the C2‒C3 (v33) and about C3‒N4 (v32) bonds occurring at 89.7 and 172.1 cm‒1 (Table S7), compared to 98(20) and 162(25) found in relative intensity measurements. The α-constants of these two excited states are predicted to be ‒125.7, 23.6, and 12.7 MHz for v33 and ‒62.0, 20.8, and 10.7 for v32. The experimental values in Table 6 are much less than this, but follow the same trend as the theoretical values. This is the basis for the assignments to individual excited states in Table 6. Interestingly, the α’s of the second excited state of v33 have almost exactly twice the values of those of the first excited state indicating that this mode is essentially harmonic. Assignment of the Ground Vibrational State of VIII. The CCSD energy of this conformer is 1.52 kJ/mol higher than the energy of I, and its Boltzmann factor is 0.46. µb is the predominating dipole moment component calculated to be 1.437 D (Table 2). Searches for strong b-type Q-branch lines led to the first assignments. bR-branch and aR-branch lines were then found and included in the fit, which were gradually extended to include aR-branch lines up to J = 26, b

Q-branch transitions with a maximum value of J = 77, bP-branch members with Jmax = 77, and

b

R-branch lines up to J = 80. Attempts to find c-type lines were also made, but these transitions

are so weak due to a small µc (Table 2) that unambiguous assignments were not possible to obtain. The spectrum consisting of 947 transitions are listed in Table S30, whereas the spectroscopic constants are shown in Table 7. The extensive spectrum made it possible to determine all sextic centrifugal distortion constants. The CCSD rotational constants are again larger than the r0-rotational constants (Table 7) by 0.97, 0.41, and 0.32 % in the cases of A, B, and C, which is expected. The experimental quartic centrifugal distortion constants are larger than the B3LYP constants. The B3LYP sextic 18 ACS Paragon Plus Environment

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centrifugal distortion constants generally deviate so much from their experimental counterparts that a discussion is not warranted. Inspection of Table 2 reveals that the rotational constants of VIII differ so much from the rotational constants of the other 17 rotamers that an unambiguous conclusion can be drawn on this basis alone. Vibrationally Excited State. 506 transitions (Table S31) of one vibrationally excited state were assigned in the same manner as described in the previous paragraph for the ground vibrational state. Maximum value of J is 75 in this case. All sextic centrifugal distortion constants were determined for this excited state, as shown in Table 7. Interestingly, some of the centrifugal distortion constants of the excited state vary much from the ground-state values. This is especially true for DK and HK The B3LYP α-constants are 392.4, ‒2.95, and ‒5.39 MHz, respectively, for v33, whose harmonic frequency is 95.3 cm‒1 (Table S8), similar to the rough experimental value (103(20) cm‒1). These α-values have the correct order of magnitude and correct signs with respect to the experimental values shown in Table 7. Searches for Further Conformers. About 4400 transitions have been assigned for the four rotamers. This includes the vast majority of the strongest transitions. Many lines with intermediate intensity have also been assigned. However, a large number of mostly weak or very weak transitions have not been accounted for. It is assumed that many of them belong to unassigned spectra of vibrationally excited forms of the four conformers, but they could also belong to conformers whose spectra have not been assigned. Extensive searches for the most likely candidates were performed. 19 ACS Paragon Plus Environment


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Rotamer IV, which is one of these candidates, is predicted to have a CCSD energy of 2.47 kJ/mol (Table 2) and hence a Boltzmann factor of 0.28 relative to I. This form was not found despite considerable efforts. It is of course a possibility that the energy difference is larger than this value resulting in lower intensity than expected for its spectrum. The fact that its largest dipole moment component µb is not higher than 1.32 D (Table 2) is a disadvantage for the assignment of this species because intensities are proportional to the square of the dipole moment component. The spectra of V (3.32 kJ/mol; Boltzmann factor = 0.20) and XII (3.84 kJ/mol; Boltzmann factor = 0.14) could not be found for reasons similar to those described in the previous paragraph. It is not very surprising that spectra of XVIII (5.00 kJ/mol), XI (5.04 kJ/mol), XV (5.79 kJ/mol) have not been identified because their Boltzmann factors are all less than 0.08.

DISCUSSION The previous study of FCH2CH2NHNH21 showed that the fluorine atom acts as a proton acceptor in internal H bonds. The present investigation shows that the hydrazino group is capable of forming H bonds not only with the most electronegative of all elements, but with π-electrons as well with strengths that are similar to those found in the fluorine compound. Conformers I, VI, VII, and VIII of 2PH, whose spectra were assigned in this work, have one thing in common: They are all stabilized by internal H bonds. I, VII, and VIII are also the three most stable forms according to the CCSD calculations (Table 2). The fourth species, VI, is


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only marginally less stable than IV, whose spectrum was not assigned. The energy difference is a mere 0.02 kJ/mol (Table 2). The reason why I has a lower energy than V, VII, and VIII needs explanation. In a complicated compound like 2PH, there will be many nonbonded interactions and explaining subtle energy differences are by no means a simple undertaking. Below factors that might be of importance in these cases are pointed out. The smallest energy difference (0.33 kJ/mol; Table 2) is found between I and VII. Conformer I has only one H bond (H12; Table 3), whereas VII has two, namely, H11 and H13, which are sharing the π-electrons. The orientation of the heavy atoms is similar in the two conformers (Table 1), but the positions of the H atoms of the hydrazino group are different. N5 is relatively close to C2 and the π-electrons of the double bond in both cases (Tables S1 and S7, respectively). The lone electron pair of N5 is favorably oriented with respect to the π-electrons in I pointing away from them. This is not the case in VII where the lone pair and the π-electrons are in much closer proximity. This is perhaps one reason why the two H bonds in VII do not fully outweigh the destabilizing influence of the lone-pair-π-electrons repulsion. Conformer VIII is 1.52 kJ/mol higher in energy than I. Both these rotamers have one H bond each, but the H atoms involved are different. In VIII, H11 forms the H bond, whereas H12 has this role in I. H11 of VIII is much closer to C2 (260.6 pm; Table 3) than H12 of I (287.9 pm; Table 3). The –NH2 part in VIII is rotated almost as far away from the π-electrons (Table S8) as it can get resulting in a minimal interaction between this group and H2C=CH-part. However, a short distance between H11 and C2 and a favorable orientation of the –NH2 part is obviously not enough to give VIII a lower energy than I. Perhaps the π-electrons in I and H12 have a better 21 ACS Paragon Plus Environment


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orientation for interaction with one another than what is the case in VIII. H12 of I may also get help from H13 which is not much further apart (about 2pm; Table S1) from C2 than H12 is. This presumed more favorable orientation may perhaps explain why I is 1.52 kJ/mol lower in energy than VIII. Conformer VI has a two H bonds with the π-electrons involving H11 and H13. In spite of this, VI has a 2.49 kJ/mol higher energy than I (Table 3). An essential difference between VI and the three other forms discussed here is the C1‒C2‒C3‒N4 dihedral angle which is 8.4° in VI (Table 1), whereas this angle is roughly 120° in the three other conformers. The C2‒C3‒N4‒H11 dihedral angle of ‒58.6° brings the H11 atom into a favorable position. The orientation of the – NH2 moiety is also favorable for the interaction of H13 with the π-electrons, but the lone-pair of the N5 atom is quite close to these π-electrons with repulsion as a result. This could be one reason why I is lower in energy than VI. The theoretical calculations revealed strengths and weaknesses of the CCSD and B3LYP quantum chemical methods. The CCSD calculations provided structures whose rotational constants always were in good agreement with the observed rotational constants of the four conformers. The same can be said about the dipole moments and the relative energies. All this was expected for this advanced method. The B3LYP calculations provided structures (not given), dipole moments (not given) and relative energies (Table 2) that were of a good quality. This method predicts quartic centrifugal distortion constants that on the average deviate by more than 15% from the experimental constants. The B3LYP vibration-rotation constants are only predicted on an order of magnitude basis, whereas the sextic centrifugal distortion constants are generally unreliable. Moreover, the 22 ACS Paragon Plus Environment

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differences between the r0- and re-rotational constants are not accurate. The last three types of parameters depend on the calculation of the third derivatives of the potential energy, which is very demanding. B3LYP is obviously not refined enough to provide very precise values of these parameters in the case of 2PH.

ASSOCIATED CONTENT Supporting Information Results of the theoretical calculations, including B3LYP/cc-pVTZ harmonic and anharmonic vibrational frequencies, quartic and sextic centrifugal distortion constants, vibration rotation constants, re- and r0-rotational constants, and nuclear quadrupole coupling constants of the two 14

N nuclei. CCSD electronic energies, structures, rotational constants, and dipole moments.

Microwave spectra of the ground state of four conformers and of vibrationally excited states of three rotamers. (PDF) This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding author *Phone: +47 2285 5674. Fax: +47 2285 5441. Emial: harald. [email protected].

Notes 23 ACS Paragon Plus Environment


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank Osamu Sekiguchi for performing a mass spectrometry investigation of 2PH. This work has been supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30). It has 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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Figure 1. Newman projections of 2-propenylhydrazine viewed along the N‒N bond.



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Figure 2. The Inner conformers of 2-Propylenylhydrazine. CCSD electronic energies in kJ/mol relative to the energy of conformer I (global minimum) are given in parentheses, The MW spectra of I, VI, VII, and VIII were assigned; see text.


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Figure 3. The Outer conformers. The CCSD energies in parenthesis are relative to conformer I. The spectra of none of these rotamers were assigned.


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Intensity (Arbitrary units)


76000

76050

76100

76150

Frequency (MHz)

Figure 4. RFMWDR spectrum of the J = 135 ← 125 pair of transitions of conformer I. The RF frequency was 5.11 MHz. The pair at the left belongs to the ground vibrational state. The pair with frequencies just below 76.1 GHz are signals from the first excited state of the torsion about the C2‒C3 bond (v33), whereas the pair at the right belongs to what has tentatively been assigned as the second excited state of v33; see text. The lone line at about 76.080 GHz is spurious and do not belong to the J = 135 ← 125 transition of conformer I.


Microwave and Quantum Chemical Study of Intramolecular Hydrogen

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