Microwave and Quantum-Chemical Study of Conformational

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Microwave and Quantum Chemical Study of Conformational Properties and Intramolecular Hydrogen Bonding of 2-Hydroxy-3-Butynenitrile (HC#CCH(OH)C#N) Harald Møllendal, Svein Samdal, and Jean-Claude Guillemin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5112923 • Publication Date (Web): 05 Jan 2015 Downloaded from http://pubs.acs.org on January 12, 2015

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The Journal of Physical Chemistry

Microwave and Quantum Chemical Study of Conformational Properties and Intramolecular Hydrogen Bonding of 2-Hydroxy-3-Butynenitrile (HC≡CCH(OH)C≡N)

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 11, 2014

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ABSTRACT: The microwave spectra of 2-hydroxy-3-butynenitrile (HC≡CCH(OH)C≡N) and a deuterated species, HC≡CCH(OD)C≡N, have been investigated in the 38 – 120 GHz spectral region. Three rotameric forms each stabilized by intramolecular hydrogen bonds are possible for this compound. The hydrogen atom of the hydroxyl group is bonded to the -electrons of the alkynyl group in one of these conformers, to the -electrons of the cyano group in the second rotamer, and to both these groups simultaneously in the third conformer. The microwave spectrum of the parent and deuterated species of last-mentioned form has been assigned and accurate values have been determined for the rotational and quartic centrifugal distortion constants of these species. The spectra of two vibrational excited states of this conformer have also been assigned and their frequencies determined by relative intensity measurements. Quantum chemical calculations at the MP2/cc-pVTZ and CCSD/cc-pVQZ have been performed to assist the microwave work. The theoretical predictions are generally found to be in good agreement with observations.

Key words: Rotational constants, centrifugal distortion constants, vibration-rotation constants vibrationally excited states, deuterated species, MP2 calculations, CCSD calculations, vibrational frequencies, radiofrequency microwave double resonance


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INTRODUCTION Intramolecular hydrogen bonding has for a long time been a favorite research theme of the Oslo laboratory and a number of hydrogen bonds with a wide variety of hydrogen donors and acceptors have been investigated over the years. In the past several years, we have reported microwave (MW) spectra of the following molecules having internal hydrogen bonds: 2isocyanoethanol

(HOCH2CH2N≡C),1

2-aminopropionitrile

(H2NCH(CH3)C≡N),2

(2-

chloroethyl)amine (ClCH2CH2NH2),3 (chloromethyl)phosphine (ClCH2PH2),4 propargylselenol (HC≡CCH2SeH),5 (CF3CH2SH),7

2-propene-1-selenol 3-butyne-1-selenol

(H2C=CHCH2SeH),6

2,2,2-trifluoroethanethiol

(HSeCH2CH2C≡CH),8

4-pentyn-1-ol

(HOCH2CH2CH2C≡CH),9 Z-3-mercapto-2-propenenitrile (HSCH=CHC≡N),10 Z-3-amino-2propenenitrile

(H2NCH=CHC≡N),11

3-butyne-1-thiol

(HSCH2CH2C≡CH),12

(methylenecyclopropyl)methanol (H2C=C3H3CH2OH),13 2-chloroacetamide (ClCH2CONH2),14 and cyclopropylmethylselenol (C3H5CH2SeH).15 References to earlier work of us and others are found in these papers, as well as in several reviews.16-20 The cyanohydrin 2-hydroxy-3-butynenitrile (HC≡CCH(OH)C≡N), henceforth referred to as HBN, is chosen for study this time. It is well established that the -electrons of nitrile (R‒ C≡N) and alkynyl groups (R‒C≡C‒R’) can act as acceptors in intramolecular hydrogen bonds where an alcohol group is proton donor. Examples include the nitrile HOCH2CH2C≡N,21 and the cyanohydrins HOCH2C≡N,22 CH3CH(OH)C≡N,23,24 and (CH3)2C(OH)C≡N,25 as well as the alkynes

HOCH2C≡CH,26

H3CCH(OH)C≡CH,27

HOCH2CH2C≡CH,28,29

and

HOCH2CH2CH2C≡CH.9 The situation in HBN is more complex than in these compounds because both the nitrile and the alkynyl groups can be involved in internal hydrogen bonding.



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Rotation about its C‒O bond may in fact lead to the three rotameric forms that are depicted in Fig. 1 with atom numbering indicated on conformer I. The H7‒C2‒O8‒H9 dihedral angle can conveniently be used to describe the conformational isomerism. This angle is about +60º in I, approximately 180º in II, and near ‒60º, in III. One intramolecular hydrogen bond between H9 and the -electrons of the C1≡N6 triple bond is present in I. A similar situation is found in III, where H9 is bonded to the -electrons of C3≡C4 triple bond. In conformer II, H9 is bonded to both these triple bonds at the same time. It should be noted that HBN is chiral and exists in the mirror-image R- and S-configurations, whose corresponding conformers have identical MW spectra. Figure 1 shows the molecule in the Sconfiguration. There is another important reason for undertaking a study of HBN: Cyanohydrins are versatile building blocks in organic synthesis,30-32 but only a few gas-phase conformational and structural studies have been reported for them.22-24 Further studies, such as the present, might help us understand better the chemical behavior of this important functional group. It should also be mentioned that cyanohydrins are of astrochemical interest. It has already been shown that the simplest cyanohydrin, hydroxyacetonitrile (HOCH2CN), is formed in astrophysical-like conditions from formaldehyde (H2C=O) and hydrogen cyanide (HC≡N).33 Formally, HBN can be considered to be a hydrogen cyanide adduct to propynal (HC≡CCHO). Propynal is an interstellar compound,34-36 which is also the case for the hydrogen cyanide molecule, which is ubiquitous in the Universe.37 The present study of its rotational spectrum should be very helpful for a potential future identification of interstellar HBN.


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The several interesting aspects of HBN motivated this first MW investigation. MW spectroscopy was chosen as our method of investigation due to its unsurpassed accuracy and resolution, which is ideal for this type of studies. The MW investigation is assisted by advanced quantum chemical modeling, which is very useful for the assignment of complex MW spectra because rather accurate values of spectroscopic constants can be predicted in this manner and be of considerable use in the assignment procedure. Information about parameters that cannot be obtained experimentally is also derived from these calculations and this allows a more profound analysis of the problem at hand.

EXPERIMENTAL SECTION

Scheme 1 Synthesis. A racemic mixture of HBN has been synthesized (Scheme 1) as previously reported38,39 with some small modifications. In a 250 mL three necked flask equipped with a

magnetic stirring bar, a nitrogen inlet and two dropping funnels, were introduced dry powdered sodium cyanide (0.163 mol, 8.0 g) and dry diethyl ether (125 mL). The suspension was cooled at 0°C and 1 mL of glacial acetic acid was added. Propiolaldehyde40,41 (0.10 mol, 5.4 g) in 10 mL of diethyl ether on one side and glacial acetic acid (8.0 mL) on the other side were introduced simultaneously via the two dropping funnels in the course of 20 min. The suspension, stirred at room temperature for 5 ACS Paragon Plus Environment


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5 h, turned brown. It was then filtered and the solid washed with 50 mL of ether. The yellow solution was washed with water (4 x 20 mL) and brine (1 x 20 mL) before drying over magnesium sulfate and evaporation of the solvent in vacuo. Purification was performed by slow distillation on a vacuum line (0.1 mbar) with a gentle heating up to 40°C of the yellowbrown liquid and selective trapping of the colorless cyanohydrin in a trap immersed in a bath cooled at ‒15°C. Yield: 5.67 g (70 mmol, 70 %). This cyanohydrin can be stored for months at ‒ 20°C. 1H NMR (CDCl3, 400 MHz)  2.80 (d, 1H, 4JHH = 2.6 Hz, CCd, 1H, 3JHH = 6.8 Hz, OH), 5.23 (dd, 1H, 3JHH = 6.8 Hz, 4JHH = 2.6 Hz, CHO). 13C NMR (CDCl3, 100 MHz)  50.7 (1JCH = 157.5 Hz (d), OCH), 75.3 (2JCH = 51.3 Hz (d), CCH), 76.8 (1JCH = 257.5 Hz (d), CCH), 115.8 (CN). The deuterated species HC≡CCH(OD)C≡N was produced by conditioning the MW cell with heavy water and then introducing the parent species. This resulted in roughly 50 % exchange of the hydrogen atom of the hydroxyl group with deuterium. Spectroscopic Experiments. HBN's vapor pressure is roughly 25 Pa at 22 ºC. The spectrum was recorded at a pressure of 5 – 10 Pa. The samples of HBN were stored in a freezer at ‒80º C. They had to be warmed up to room temperature in order to fill the MW cell with fresh sample. During this process, the compound decomposed partly to propynal and hydrogen cyanide, both of which were identified by their reported MW spectra.42-46 The intensity of the spectrum of propynal increased by a factor of 3 in the cell in the course of 20 minutes, which showed that the decomposition continued in this environment possibly catalyzed by the cell walls. The cell was therefore filled frequently with fresh portions of HBN. The MW spectrum was recorded using the Stark MW spectrometer of the University of


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Oslo described in details elsewhere.47 Only salient features are reported here, namely, the accuracy of this spectrometer, which is 0.10 MHz for isolated lines and the resolution, which is about 0.5 MHz for strong transitions. Measurements were made in the 38 – 120 GHz spectral region. Radio-frequency microwave double-resonance (RFMWDR) spectra48 were recorded to obtain unambiguously assignments of selected transitions.

RESULTS Quantum Chemical Calculations. Frozen-core MP249 and CCSD50-53 computations were executed using the Abel cluster of the University of Oslo. The MP2 calculations were performed with the Gaussian 09 suit of programs,

54

while the CCSD computations were done employing

the Molpro program55 observing the default convergence criteria of the two programs. The correlation-consistent cc-pVTZ triple- and the cc-pVQZ quadruple- basis sets56 were employed in the MP2 and the CCSD calculations, respectively. A MP2/cc-pVTZ potential function for rotation of the hydroxyl group (Figure 2) was obtained by stepping the H7‒C2‒O8‒H9 dihedral angle (see Fig. 1) in 10º intervals with all remaining structural parameters allowed to vary freely. This function has three minima corresponding to conformers I – III. The MP2 structure of each conformer was optimized with the results given in the Supporting Information, Tables S1 – S3. The MP2 computations predict that the global energy minimum occurs for II at a H7‒C2‒O8‒H9 dihedral angle of 185.7º (‒ 174.3º), whereas this angle is 59.6º for I, and 284.4º (‒75.6º) for III. Conformer II has an MP2 electronic energy that is 6.91 kJ/mol less than the energy of I, and 6.65 kJ/mol less that that of III. Corrected for zero-point vibrational effect these differences become 6.30 and 5.79 kJ/mol, respectively. The potential function has three maxima (transition states). The characteristics of 7 ACS Paragon Plus Environment


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these transition states were calculated at the MP2 level with the results displayed in Tables S4 – S6 of the Supporting Information. The transition states are located at 111.3, 256.2, and 355.4º for the H7‒C2‒O8‒H9 dihedral angle with electronic energies that are 10.47, 7.16, and 12.10 kJ/mol higher than the energy of the global minimum. MP2 calculations were also undertaken to obtain harmonic and anharmonic vibrational frequencies, the vibration-rotation constants (the 's),57 Watson quartic and sextic centrifugal distortion constants,58 r0 and re rotational constants, and dipole moments (see Tables S1 – S3). The procedure recommended by McKean et al59 was followed when calculating the 's and the centrifugal distortion constants. Optimized CCSD/cc-pVQZ structures of I – III are listed in Table 1. Further details of the CCSD calculations are listed in Tables S7 – S9 of the Supporting Information. Table 2 contains the rotational constants calculated from the CCSD structures, the MP2 quartic centrifugal distortion constants,58 CCSD dipole moments, and CCSD electronic energy differences. A few remarks on the CCSD structures are in order. The O8‒H9 bond length is 95.8 pm in all forms of HBN (Table 1) compared to the equilibrium O‒H bond length in methanol, which is 95.6 pm.60 A slight elongation of this bond length from its value in methanol is expected for the title compound because of the internal hydrogen bonds in the three conformers. The CCSD bond lengths of the C1≡N6 triple bond are 115.1 – 115.0 pm, while re = 115.54 pm has been reported for CH3CN for its C≡N bond.61 The re value of the C3≡C4 triple bond in acetylene is 120.289 pm,62 slightly longer than 119.9 – 120.0 pm found in the present case. The CCSD method predicts II to be the global minimum (Table 1). The electronic energies of I and III are 6.08 and 5.73 kJ/mol higher, respectively. These values are very similar 8 ACS Paragon Plus Environment

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to the MP2 results above. The lower energy of II compared to I and III may reflect that H9 is simultaneously bonded to two -electron systems in II, whereas only one hydrogen bond is present in the two other conformers. The prediction that I and III have nearly the same energy indicates that the hydrogen bonds of these two conformers have similar strengths. Assignment of the Ground-State Spectrum of II. The decomposition of HBN to propynal and hydrogen cyanide was a severe complication because propynal has a very strong MW a-type R-branch spectrum as well as a weaker b-type spectrum resulting in a dense spectrum in the entire investigated spectral interval (38 – 120 GHz). Propynal has also several low vibrational frequencies and MW spectra of excited states of these vibrations added to the spectral richness. All this resulted in numerous overlaps with transitions of HBN. It was also unfortunate that most transitions belonging to propynal are much stronger than those of HBN. Another negative factor was the decomposition of HBN, which resulted in a relatively rapid increase in the intensity of the propynal lines, was accompanied by a simultaneous reduction of intensities of the HBN transitions. It is seen from Table 2 that the lowest-energy conformer II has a comparatively large a component of about 3.2 D, while b and c are much smaller. We therefore concentrated on finding a-type R-branch lines of this conformer using the rotational and quartic centrifugal distortion constants of Table 2 to predict their approximate frequencies. RFMWDR searches for selected transitions in the frequency region above 75 GHz soon met with success. A typical example of a RFMWDR identification is exemplified by the J = 2311 ← 2211 pair of transitions shown in Fig. 3. The frequencies of transitions having values of the pseudo quantum number K‒1 larger



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than about 10, which occur in this spectral region, could now be predicted rather precisely. These transitions appear as coalescing pairs with very rapid Stark effects and this property was useful to obtain a secure assignment. Further assignments were then gradually made. A total of 275 aRtransitions listed in Table S10 of the Supporting Information were ultimately assigned and leastsquares fitted to Watson's Hamiltonian in the S-reduction Ir-representation form,58 employing Sørensen's program Rotfit.63 The resulting spectroscopic constants are shown in Table 3. The maximum value of J is 24 and the maximum value of K‒1 is 23. None of the transitions displayed a hyperfine structure due to quadrupole coupling of the

14

N nucleus. Searches for b- and c-type

lines were made, but none could be unambiguously assigned. This is not surprising because the corresponding CCSD dipole moment components are rather small, being 0.51 and 0.08 D, respectively, resulting in very weak transitions. Accurate values of the rotational and quartic centrifugal distortion constants have been obtained (Table 2). Attempts were made to include sextic centrifugal distortion constants in the least-squares fits, but the resulting constants had so large standard deviations that it was decided to limit the fits to include only quartic centrifugal distortion constants. The spectroscopic constants shown in Table 3 should predict the frequencies of rotational transitions that occur outside the investigated spectral interval (38 – 120 GHz) with a high degree of precision. The CCSD rotational constants in Table 1 of the three conformers have similar values with one exception, namely, the A rotational constant of II that differs from the two other A constants by more than 100 MHz. The experimental effective (r0) A constant (Table 2) is much closer to the CCSD A constant of II than to the corresponding constants of I and III. It is therefore certain that the spectrum of Table S10 indeed belongs to II. The rotational constants of the deuterated species HC≡CCH(OD)C≡N discussed below confirm this assignment. 10 ACS Paragon Plus Environment

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The effective (r0) ground-state rotational constants (Table 2) are smaller than the CCSD constants (Table 1) by 16.7, 7.2, and 6.5 MHz in the cases of A, B, and C, respectively. The CCSD rotational constants are approximations of the equilibrium counterparts, which are usually found to be smaller than the effective constants since r0 bond lengths are normally longer than re bond lengths resulting in larger principal moments of inertia and smaller values for the effective rotational constants. The MP2 method predicts these differences to be 14.0, 5.9, and 7.6 MHz (Table S2) in fair agreement with the present findings. There is very good agreement between the experimental (Table 3) quartic centrifugal distortion constants and the MP2 equivalents (Table 2) with one exception, namely, d2, but this experimental parameter is the least accurate centrifugal distortion constant. Vibrationally Excited States. The RFMWDR spectrum revealed transitions belonging to several vibrationally excited states. The spectra of two of these were assigned in the same manner as discussed above for the ground-state spectrum. The spectroscopic constants are included in Table 3 and the spectra are found in the Supporting Information, Tables S11 and S12. The vibration-rotation -constants57 found by subtraction of the excited-state rotational constants from their ground-state equivalents are A = ‒9.302(88), B = ‒9.5343(88), and C = ‒0.401 MHz, compared to the MP2 values ‒11.2, ‒8.7, and ‒0.2 MHz, respectively, calculated for the lowest bending vibration (Table S2). Rough relative intensity measurements yielded 110(25) cm‒ 1

for this vibration compared to the anharmonic MP2 frequency, which is127 cm‒1 (Table S2). A = ‒10.70(14), B = 6.5467(98), and C = ‒2.670 MHz are calculated in a similar

manner from the entries in Table 3 for the other excited state. The corresponding MP2 parameters of the second lowest bending fundamental are ‒16.0, 7.6, and ‒3.1MHz, respectively. Relative



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intensity measurements yielded ca 180 cm‒1 for this vibration in accord with an anharmonic fundamental of 195 cm‒1 (Table S2). It is concluded that the MP2 calculations in this case is able to predict correct sign and order of magnitude for the -constants. Deuterated Species. The spectrum of the deuterated species HC≡CCH(OD)C≡N, which was assigned in a straightforward manner, is listed in Table S13 and the spectroscopic constants are displayed in Table 3. The substitution coordinates64 of the hydrogen atom of the hydroxyl group in the principal axis system were calculated from the rotational constants of the parent and deuterated species (Table 3) using Kraitchman's equations65 and found to be │b│ = 158.651(51) and │c│ = 113.542(51) pm, while the a-coordinate has a small imaginary value. The uncertainties (one standard deviation) have been calculated from the standard deviations of the rotational constants. The CCSD values of these coordinates are │a│ = 8.39, │b│ = 157.52, and │c│ = 113.89 pm (Table S8), in good agreement with the substitution coordinates above. The corresponding CCSD values of conformer I are │a│ = 78.85, │b│ = 213.39, and │c│ = 1.39 pm (Table S7), while the values of III are │a│ = 77.17, │b│ = 208.89, and │c│ = 16.54 pm (Table S9). The substitution coordinates of the hydroxyl group again show conclusively that the assigned spectra belong to II and that confusion with I or III is out of the question. Searches for the Spectra of I and III. Conformer III has a dipole moment component along the a-principal inertial axis as large as 4.4 D according to the CCSD method (Table 2). Searches were performed for selected aR-transitions of this rotamer using the RFMWDR technique, but no characteristic double-resonance signals were observed. This is taken as an indication that there is a substantial energy difference between III and II producing insufficient intensities for the spectrum of III. This is in accord with the MP2 and CCSD calculations, which


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predicts an energy difference of about 6 kJ/mol. Searches for I were also negative. This form is also significantly higher in energy than II as discussed above.

DISCUSSION Several intramolecular forces appear to determine the conformational properties of HBN. Internal hydrogen bonding is present in all three forms. The hydrogen bonds are composed primarily of two types of interaction, namely, dipole-dipole interaction and covalent interaction between the H9 atom and -electron systems of the triple bonds. There are three polar groups in HBN that may interact. The most polar of them is the cyano group, which has a bond moment as large as 3.6 D66 with nitrogen as the negative end. The bond moment of the hydroxyl group is 1.5 D66, while propyne (CH3C≡CH) has a dipole moment of 0.7804 D with CH as the negative end.67 In conformer I, the CCSD angle between the C1≡N6 and the O8‒H9 bonds is 67.8º (from Cartesian coordinates in Table S7). The two groups are oriented in such a manner that dipoledipole stabilization should be significant. The O8‒H9 and the C3≡C4 bonds are 3º from being parallel, while the associated bond moments are antiparallel resulting in a minor repulsion. The non-bonded distances between the H9 atom and the C1 and N6 atoms are 256 and 340 pm, respectively (Table S7) compared to the sum of the Pauling van der Waals radii68 of hydrogen (120 pm) and the half-thickness of an aromatic molecule (170 pm), which is 290 pm. This suggests that the covalent stabilization between H9 and the -electrons of the C1≡N6 bond is not a large effect. Conformer III resembles I. The bond moments of the O8‒H9 and the C3≡C4 groups stabilize III in this case, while the electrostatic interaction caused by the O8‒H9 and C1≡N6 13 ACS Paragon Plus Environment


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groups destabilizes III. The non-bonded distance between H9 and C3 is 240 pm and the distance between H9 and C4 is 334 pm resulting in a weak covalent stabilization. In conformer II, the electrostatic interactions between the O8‒H9 group and both the C1≡N6 group and the C3≡C4 group are similar to those found in I and III, respectively. The covalent stabilization with the -electrons of the two groups is also similar. The much better situation for both electrostatic and covalent of the hydrogen bonding in II makes this conformer several kJ/mol lower in energy than I and III in agreement with the present experimental findings.

ASSOCIATED CONTENT Supporting Information Results of the theoretical calculations, including electronic energies; molecular structures; dipole moments; harmonic and anharmonic vibrational frequencies; rotational and centrifugal distortion constants; and rotation-vibration constants. Microwave spectra of the ground and vibrationally excited states of the parent species; and of the ground state of the deuterated species. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +47 2285 5674; Fax: +47 2285 5441; E-mail: [email protected]


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ACKNOWLEDGEMENTS We thank Anne Horn for her skillful assistance. 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 French National Program Physique et Chimie du Milieu Interstellaire (PCMI (INSU-CNRS)) and the Centre National d’Etudes Spatiales (CNES) for grants.



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Figure 1. Models of three conformers of 2-hydroxy-3-butynenitrile. Atom numbering is given on conformer I. The MW spectrum of II was assigned.


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-1

12

Relative energy / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

4

0 0

40

80

120

160

200

240

280

320

360

Dihedral angle / deg

Figure 2. Relative MP2/cc-pVTZ electronic energy as a function of the H7‒C2‒O8‒H9 dihedral angle.



2

J = 23

22

0

Intensity

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-2

116220

116235

116250

116265

Frequency / MHz

Figure 3. RFMWDR spectrum of the J = 2311 ← 2211 pair of transitions. The RF was 5.85 MHz. The frequencies are 116237.69 and 116247.76 MHz for the 2311,13 ← 2211,12 and 2311,12 ← 2211,11 transitions, respectively. The intensity is in arbitrary units.


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Table 1. CCSD/cc-pVQZ Structures of the Conformers I, II, and III of HC≡CCH(OH)C≡N Conformer:

I

II

III

Bond distance (pm) C1–C2

148.5

148.5

147.7

C1–N6

115.1

115.1

115.0

C2–C3

146.6

147.1

147.1

C2–H7

109.3

108.9

109.3

C2–O8

140.6

140.3

140.8

C3–C4

119.9

120.0

120.0

C4–H5

106.2

106.2

106.2

O8–H9

95.8

95.8

95.8

Angle (deg) C1–C2–C3

110.5

109.9

110.7

C1–C2–H7

106.7

107.2

106.7

C1–C2–O8

111.3

111.4

107.3

C3–C2–H7

108.3

109.1

108.3

C3–C2–O8

108.9

113.2

112.9

H7–C2–O8

111.1

105.8

110.9

C2–O8–H9

108.5

108.6

107.8

C2–C1–N6

178.1

178.5

179.4

(Table 1 continues next page)



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Page 20 of 33

(Table 1 continued)

C2–C3–C4

178.4

177.0

176.7

C3–C4–H5

179.4

179.9

179.0

Dihedral angle (deg) C1–C2–O8–H9

–59.7

70.0

168.8

C3–C2–O8–H9

178.2

–54.5

46.7

H7–C2–O8–H9

59.0

–173.8

–75.1

N6–C2–O8–H9

–58.5

70.0

169.4

C4–C2–O8–H9

177.9

‒54.3

45.6

H5‒C2‒O8‒H9

177.9

‒54.3

45.5


Page 21 of 33

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Table 2. Theoretical Parametersa of Spectroscopic Interest of Conformers I, II, and III of HC≡CCH(OH)C≡N conformer:

I

II

III

Rotational Constants (MHz) A

5909.8

5773.6

5938.5

B

2856.6

2881.4

2852.7

C

2026.2

2038.4

2027.7

DJ

1.39

1.42

1.36

DJK

‒8.03

‒7.31

‒7.64

DK

27.2

23.8

26.6

d1

‒0.597

‒0.600

‒0.582

d2

‒0.0369

‒0.0406

‒0.0393

Dipole moment (Debyeb) a

1.83

3.23

4.38

b

1.42

0.51

1.31

(Table 2 continues next page) 21 ACS Paragon Plus Environment


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Page 22 of 33

(Table 2 continued)

c

1.69

0.08

1.43

tot

2.87

3.27

4.78

Relative electronic energyc (kJ/mol) E

a

6.08

0.0

5.73

CCSD/cc-pVQZ Rotational constants, dipole moments and relative electronic energies, and

MP2/cc-pVTZ centrifugal distortion constants.

b

1 debye = 3.33564 × 10‒30 C m.

c

Relative to

conformer II. Not corrected for zero-point vibrational effects.


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Spectroscopic Constantsa of Conformer II of HC≡CCH(OH)C≡N and HC≡CCH(OD)C≡N species: vibrational state:

parent

deuterated

ground

lowest bend.

second lowest bend.

ground

Av (MHz)

5756.892(46)

5766.194(76)

5767.59(13)

5518.987(85)

Bv (MHz)

2874.2429(40)

2883.7772(78)

2867.6962(98)

2854.6171(83)

Cv (MHz)

2031.8990(50)

2032.300(10)

2034.569(12)

2011.548(11)

DJ (kHz)

1.4360(35)

1.378(12)

1.202(10)

1.3811(81)

DJK (kHz)

‒7.358(16)

‒6.957(56)

‒4.606(43)

‒6.137(34)

DK (kHz)

20.52(61)

29.4(21)

20.8(16)

20.5(11)

d1 (kHz)

‒0.5912(34)

‒0.6211(97)

‒0.708(13)

‒0.5563(77)

d 2 (kHz)

‒0.0275(23)

‒0.0648(87)

0.142(10)

‒0.0274(45)

rmsb

1.296

1.338

1.570

1.462

Nc

275

142

138

155

(Table 3 continues next page)



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Page 24 of 33

(Table 3 continued)

a

S-reduction Ir-representation.58 Uncertainties represent one standard deviation. The spectra are

listed in Tables S10 – S13 of the Supporting Information. b Root-mean-square deviation defined as rms2 = [(obs – calc)/u]2/(N – P), where obs and calc 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. c Number of transitions used in the fit.


Page 25 of 33

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