Millimeter Wave Spectrum of the Weakly Bound Complex CH2

Article pubs.acs.org/JPCA

Millimeter Wave Spectrum of the Weakly Bound Complex CH2CHCN·H2O: Structure, Dynamics, and Implications for Astronomical Search Published as part of The Journal of Physical Chemistry A virtual special issue “Spectroscopy and Dynamics of Medium-Sized Molecules and Clusters: Theory, Experiment, and Applications”. Camilla Calabrese,† Annalisa Vigorito,† Assimo Maris,† Sergio Mariotti,‡ Pantea Fathi,§ Wolf. D. Geppert,*,§ and Sonia Melandri*,† †

Dipartimento di Chimica “Giacomo Ciamician” dell’Università degli Studi di Bologna, via Selmi 2, I-40126 Bologna, Italy INAF − Osservatorio di Radioastronomia (formerly Institute of Radioastronomy), via P. Gobetti, 101, I-40129 Bologna, Italy § Department of Physics, Stockholm University, Albanova University Center, SE-106 91 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: The weakly bound 1:1 complex between acrylonitrile (CH2CHCN) and water has been characterized spectroscopically in the millimeter wave range (59.6−74.4 GHz) using a Free Jet Absorption Millimeter Wave spectrometer. Precise values of the rotational and quartic centrifugal distortion constants have been obtained from the measured frequencies of the normal and isotopically substituted water moiety (DOH, DOD, H18OH). Structural parameters have been estimated from the rotational constants and their differences among isotopologues: the complex has a planar structure with the two subunits held together by a O−H···N (2.331(3) Å) and a C− H···O (2.508(4) Å) interaction. The ab initio intermolecular binding energy, obtained at the counterpoise corrected MP2/aug-cc-pVTZ level of calculation, is De = 24.4 kJ mol−1.

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INTRODUCTION Nitriles play a decisive role in the interstellar medium. They have been detected in different objects like dark clouds,1 lowmass protostars,2 and circumstellar envelopes.3 The heaviest aliphatic interstellar molecule detected so far is a nitrile (HC11N),4 and many different species containing a cyano group have been identified by radioastronomy observations. Furthermore, the Cassini−Huygens mission yielded that the abundances of several nitriles in Titan’s atmosphere are orders of magnitude higher5 than anticipated by pre-Cassini models.6 Among interstellar and atmospheric compounds, acrylonitrile (CH2CHCN from here on abbreviated as ACN) deserves some special interest. The compound has been identified in the Orion KL region7,8 and toward the Sagittarius B2(N) molecular nebula.9,10 In the latter environment, even excited states of acrylonitrile were detected.11 ACN has also been identified in the carbon-rich star IRC+102163 and in the cold dark cloud TMC-1.12 Due to its abundance and its many lines in the radiofrequency spectrum it is often referred to as a “weed” by astronomers. ACN and other acrylic compounds might also function as starting compounds in reaction sequences resulting in the formation of larger molecules detected in different astronomical objects. For example, the corresponding acrylonitrile cation can be involved in many different chemical © XXXX American Chemical Society

processes with interstellar molecules, which can lead to heavier carbon and nitrogen-containing ionic species.13 It has also been argued that this ion can undergo reaction sequences to form heterocyclic ring compounds like the pyrimidine cation.13 Nevertheless, the latter has not yet been observed in the interstellar medium. In the solid phase, ion irradiation and UV photolysis of ACN results in the formation of cyanoacetylene, which has been detected in the interstellar medium, comets, and Titan’s atmosphere.14 ACN is also a pivotal compound in the latter environment and its protonated form is most likely present in high abundances in the upper atmosphere of Titan, a conclusion based on a strong signal at m/z = 54 recorded by the ion-neutral-mass spectrometer (INMS) on board the Cassini spacecraft during a flyby on 2005 April 16.5,15 The compound was also found to be formed in a recent plasma discharge experiment simulating the chemistry of Titan’s ionosphere.16 Finally, ACN could be a constituent of cometary comae and ices, because another unsaturated nitrile, namely cyanoacetylene, has been detected in comets. Whereas in Hyakutake only an upper limit of the fractional abundance of Received: August 29, 2015 Revised: November 1, 2015

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DOI: 10.1021/acs.jpca.5b08426 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Block diagram of the free-jet absorption millimeter wave spectrometer (FJAMMW) with the setup based on a chain of two multipliers (doublers): 1. MicroWave synthesizer - HP8672A 2−18 GHz. 2. Reference signal, Rb oscillator 5 MHz, Ball-Efraton FRK-LLN. 3. Hittite Ampifier MW 5−20 GHz Gain: +20 dB. 4. Variable attenuator, NARDA 10 dB. 5. Frequency doubler Spacek-laboratories 26.5−40 GHz. Ouput power: +17 dBm Waveguide WR28. 6. Dorado Isolator 26.5−40 GHz, 41 GHz. 7. Low pass filter Spacek-laboratories, f max = 41 GHz. 8. Frequency doubler Spacek-laboratories 53−80 GHz. Ouput power: −3 dBm to 0 dBm, Waveguide WR12 9. Variable attenuator 0−35 dB. 10. Horn-dielectric lens system (Montech-Clayton, Australia). 11. Supersonic-jet expansion chamber. 12. Stark plates. 13. Schottky-diode detector: Millitech DXW10 for frequencies above 60 GHz or a Millitech 4731 4H-1111 for frequencies below 60 GHz. 14. High-voltage square-wave modulation: electric field up to 750 V·cm−1 at a frequency of 33 kHz. 15. Digital lock-in amplifier. 16. Experimental control and data acquisition.

cyanoacetylene of 3.5 × 10−4 could be established,17 the latter compound was unambiguously detected in Hale−Bopp with a fractional abundance of (1.9 ± 0.2) × 10−4,18 which is in the same range as the value for acetonitrile.19 Thus, the presence of ACN in comets cannot be ruled out. Due to the electron pair located at the nitrogen, nitriles are quite efficient proton acceptors and can efficiently undergo hydrogen bonding with proton donors like water, alcohols, or proton-containing cations. Water clusters of ACN should thus be readily formed and could even exist in the interstellar medium. To the best of our knowledge, no neutral hydrogenbonded van der Waals cluster has been detected in the interstellar medium so far. This includes even clusters one would deem as being likely to be present in the interstellar medium like the water dimer, whose radio-frequency spectrum was measured in the range 7−25 GHz by Coudert et al.20 Dyke and Muenter covered the region between 8 and 50 GHz21 and Fraser et al. the one between 14 and 110 GHz.22 Measurements in the submillimeter region (up to about 700 GHz) have been reported by Zwart et al.23 whereas very recently, Tretyakov et al. published a survey of the rotational spectrum of the water dimer between 100 and 150 GHz.24 The molecular constants and the tunnneling splittings of the water dimer were investigated in detail both theoretically and experimentally (with new data between 53 and 118 GHz and the data set reported in ref. 23) by Keutsch et al.25 The search for water clusters is most promising in starforming regions where the warm-up during star formation leads to evaporation of the icy mantles of the interstellar grains. Due to the higher temperatures in these environments and the consequently higher rotational excitation of these molecules, these clusters are more likely to be detected at higher frequencies probing transitions with higher J quantum numbers. Unfortunately, extrapolation of the measured low-frequency spectra to higher frequencies and especially the prediction of the intensity profiles of the high-frequency bands (which is essential to identify the molecular carrier for those) is prone to errors due to the shallow potential energy wells of these clusters and, consequently, their less rigid nature compared to that of valence-bound molecules. Due to the ro-vibrational nature of the water dimer transitions, a perfect fitting of the observed

lines is difficult and the exact prediction of the line intensity profiles at frequencies far from the measured ones is not accurate. However, the ACN−water cluster is more rigid due to the existence of two van der Waals bonds: one between the lone pair of the nitrile and one of the hydrogen atoms of the water molecule and another one between the oxygen atom and one of the hydrogen atoms bound to the terminal carbon atom of ACN. This leads to a less floppy structure and enables reliable fitting of the different quantum numbers and prediction of the intensity profiles of the cluster. Because ACN can, due to its polarity, be efficiently frozen out on interstellar grain surfaces and is one of the ingredients into interstellar ices, also ACN can be expected to be a component of ices covering those grains. During the warm-up phase associated with formation of stars ACN will be desorbed and enter the gas phase and codesorption of ACN and water in the form of clusters could be possible. Also in cometary ices the compound is likely to be a constituent due to its presence in cometary comae. During the approach of comets to the sun the compound could also evaporate from the cometary nucleus, possibly partly as an ACN−water cluster. In an experimental study, sublimation of solid pure ACN has been observed to occur at 135 K.26 As mentioned, codesorption of ACN and water or methanol from ices containing these two compounds in the form of clusters cannot entirely be ruled out and the possible presence of ACN−water clusters in star-forming regions and cometary comae is thus not completely unlikely. Unfortunately, spectroscopic data on clusters of ACN with water and alcohols widely present in the interstellar medium in the spectral region of receivers employed at present radiotelescopes is still lacking, which hampers their detection in astronomical objects. The most useful spectral region for the unambiguous identification of molecular systems is that of rotational molecular transitions (microwave and millimeter wave region), which can be considered as molecular fingerprints. In the case of weakly bound molecular complexes the rotational spectra can be studied in the gas phase generating the molecular complexes in a supersonic expansion of a monatomic carrier gas containing about few percent of the moieties. Rotational studies on molecular complexes formed by molecules containing the CN group include many complexes B


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Figure 2. Sketch of the molecular structure of the three conformers of ACN·W, and the corresponding stable structures of BZN·W and CH3CN·W, obtained at the MP2/aug-cc-pVTZ level of calculation. The binding energies and electron density level curves are also shown, in atomic units: purple 0.08−0.04, red 0.04−0.02, orange 0.02−0.01, and yellow 0.005−0.01 e a0−3, respectively.

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with HCN and a few containing CH3CN (acetonitrile).27 In the examples reported in the literature, HCN is generally a proton donor whereas CH3CN acts as a proton acceptor at the N atom. The studies on the complexes of CH3CN include H atom donors such as HF,28,29 HCl,30 HCN,31 HCCH,32 and clusters with Lewis acid electron acceptors functioning as acceptors of the lone electron pair at the cyano group, such as BF3,33 SO3,34 and surprisingly also F2.35 Recently, also the rotational spectrum of the solvated acetonitrile molecule (CH3CN· H2O) has been published.36 In all these examples the observed rotational spectrum is that originating from a symmetric top and in the case of the complex with water two tunneling components related to the motion of water were observed. Among the CN containing molecules, the rotational spectrum of several isotopologues of benzonitrile·water (BZN·W) has also been reported.37 Detailed structural information on the position of the water oxygen atom in the complex, which is also confirmed to be planar by the values of the planar moment of inertia, was obtained. As in the other cited complexes, the cyanide group acts as a hydrogen bond acceptor at the N atom but in the case of BZN·W a secondary interaction between the water oxygen and the aromatic hydrogen atom in the ortho position with respect to the cyanide group changes the hydrogen bond angle from almost collinear (such as in CH3CN·H2O) to perpendicular with respect to the CN triple bond. This paper presents a microwave spectroscopy study of the 1:1 acrylonitrile−water adduct (CH2CHCN·H2O, from here on ACN·W) in the region between 59.6 and 74.4 GHz. The scope of the work is 2-fold: on one hand, we will characterize this species in the frequency range useful for astronomic detection, and on the other hand, we will provide detailed structural information on the interaction of water with ACN.

EXPERIMENTAL AND COMPUTATIONAL METHODS The millimeter-wave (59.6−74.4 GHz) spectrum of ACN·W was recorded using a Stark modulated Free-Jet Absorption Millimeter Wave (FJAMMW) spectrometer, the basic design of which has been already described elsewhere.38,39 In addition to the 2013 design, a new radio-frequency source has been used and a box diagram of the microwave circuit and modulating scheme used in the experiment is reported in Figure 1. It is based on an X-Band synthesizer followed by a x4 multiplier chain composed of two multipliers (doublers). The first doubler is active and the second one is passive, and they are separated by an isolator and a low pass filter. The presence of the isolator improves the coupling between adjacent elements of the chain, and the filter helps to eliminate unwanted harmonic signals that could be present. In fact, the frequency multiplication process is not an ideal one because it can give rise to some disturbances (unwanted harmonic frequencies, phase noise, and even spurious signals) in the output signal, but if a high-frequency generator is not available, the introduction of filters placed downstream of the generator and each multiplier improves the performances of this kind of multiplication system. Depending on the section of the chain, both a band-pass filter and a filtering effect due to waveguide cutoff have been used. Power amplifiers as well as variable attenuators have been placed to drive correctly the multipliers and the FJAMMW. The spectrometer has a resolving power of about 300 kHz and an estimated accuracy of about 50 kHz. ACN was purchased from Sigma-Aldrich (purity >99%) and used without further purification. The compound was cooled by a mixture of ice and NaCl while water was maintained at room temperature. A stream of argon at a pressure of about 600 kPa was passed above both ACN and water, which were kept in two separate containers. We used a concentration of ACN and water in the gas mixture of about 3% and 7%, respectively. The mixture was then expanded from a stagnation pressure of 45 C


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Table 1. Spectroscopic Parameters, Relative Energies and Shape of the Three Possible Conformations for ACN·W Complex, Calculated with MP2/aug-cc-pVTZ

a Absolute energy: −246.832269 hartree. bAbsolute zero-point corrected energy: −246.758165 hartree. cThe second value corresponds to the BSSE corrected single-point calculation.

in detail by running several ab initio optimization procedures, each of them starting from a different initial geometry. Two planar forms, in which the water acts as both proton donor and proton acceptor, and a conformation characterized by an almost linear H···N hydrogen with the unbound hydrogen atom lying out of the heavy atom plane, were identified and are shown in Figure 2 with their corresponding binding energies and the electron density level curves in the ACN molecular plane. In the same picture also the calculated global minimum structures of BZN·W and the two lowest minima of CH3CN·water (CH3CN·W) are displayed for comparison. The binding energies, spectroscopic constants, and electric dipole moment components for ACN·W are listed in Table 1.

kPa to about 0.5 Pa through a 0.35 mm diameter pinhole nozzle. These settings have been found as optimal for the formation of the 1:1 complex in our experiments. Full geometry optimization and evaluation of the Hessian matrix of the monomers and dimers was carried out at the MP2/aug-cc-pVTZ level of calculation. The basis set superposition effect (BSSE) on the obtained geometries was estimated by the counterpoise correction procedure.40 All calculations were performed with the GAUSSIAN09 program package.41

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RESULTS Although a systematic conformational search was not performed, the conformational space of ACN·W was explored D


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Figure 3. Two rotational transitions recorded for ACN·W1 showing the two vibrational states 0− and 0+.

Table 2. Experimental Spectroscopic Constants (S-Reduction, Ir-Representation) of the Observed ACN·W1 Conformation and the Related Isotopologues H18OH

ACN·W1 A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz χaa/MHz χbb − χcc/MHz Nb σc/kHz

0−

0+

0−

0+

5282.020(2)a 3422.834(2) 2074.851(3)

5282.191(2) 3422.981(2) 2074.835(3)

5242.710(4) 3214.811(6) 1990.805(8)

5242.852(4) 3214.940(6) 1990.786(8)

8.70(1) −3.93(3) 15.76(4) −4.085(2) −0.756(1) −0.44(8) −3.36(4) 68

7.62(2) 0.82(5) 9.08(6) −3.46(1) −0.744(5) [−0.44]d −3.64(8) 42

29

42

24 44

DOH

DOD

5290.904(3) 3338.465(5) 2044.934(7) 8.25(2) −5.13(6) 18.01(6) −3.80(1) −0.689(7) [−0.44]d [−3.36]d 34 36

5264.760(3) 3150.127(4) 1969.736(6) 7.02(1) −1.30(4) 13.49(6) −3.155(2) −0.632(2) [−0.44]d [−3.36]d 39 39

a Error in parentheses in units of the last digit. bNumber of lines in the fit. cRoot-mean-square deviation of the fit. dValues fixed to the ones of the parent species.

accumulated to obtain more accurate data and to extend the measurements also to weaker μa-type rotational transitions originating from higher rotational quantum numbers. All the observed rotational transitions were seen as doublets, consisting of a stronger low-frequency component and a weaker highfrequency component (as shown in Figure 3), with the same intensity ratios of 3:1 for all observed doublets. In some of the measured transitions the nuclear-quadrupole hyperfine structure, related to the presence of the nitrogen 14N nucleus was resolvable. The list of the observed frequencies is available as Supporting Information. To increase information about the structure of this molecular complex, isotopologues of the water clusters were investigated namely: ACN-H18OH, ACN-DOD, and ACN-DOH. These species were generated by adding D2O or H218O to the system instead of normal water, and observing the rotational spectrum in the same conditions as for the normal species. The presence of a second monodeuterated isotopologue, with the deuterium atom not involved in the intermolecular interaction, was not found in the rotational recorded spectrum probably due a lower abundance than for the other monodeuterated species because of its higher zero point energy. The list of measured frequencies for all observed isotopologues is available as Supporting Information. The observed doubling of the transition lines observed for the parent species was also seen for the ACN·H18OH isotopologue whereas, in the case of the deuterated

It can be seen in Figure 2 that both planar conformers are stabilized by a stronger water hydrogen···nitrile interaction (identified by the orange area between the two moieties) and a weaker oxygen···hydrogen bond (yellow area) involving terminal and nonterminal alkene hydrogen atoms in ACN· W1 and ACN·W2, respectively. Stabilization energy values show that the water moiety can maximize the intermolecular interaction with ACN in the first conformation (ACN·W1). Although the electron density plots show that the stronger intermolecular bond takes place between the water hydrogen atom and the nitrogen lone pair (red area in ACN·W3), due to the cooperation of two weaker interactions, similar and higher binding energies are achieved by conformers ACN·W2 and ACN·W1, respectively. A conformation similar to ACN·W1 was calculated to be the global minimum also for BZN·W and was indeed observed experimentally37 whereas the linear arrangement (similar to ACN·W3) is calculated to be the lowest energy conformation for CH3CN and this result was also confirmed experimentally.29 Based on the rotational constants and dipole moment components reported in Table 1, the prediction of the rotational spectrum of the most stable conformation, ACN· W1, was obtained in the frequency range covered by the FJAMMW spectrometer. In the first recorded scan, several μbtype transitions were observed with rotational quantum number J ranging from J = 6 up to J = 15, and assigned to the global minimum conformation (ACN·W1). Then many scans were E


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simulations performed could be used for attempts of astronomic observations of the ACN·W cluster. A problem could arise from the existence of at least three different conformers found by the ab initio calculations, although no evidence of more than one structure of the ACN·W cluster has been detected in the present study and all the observed frequencies have been assigned to the global minimum geometry (ACN·W1). It cannot be excluded that other structures of this complex are present in the interstellar medium and higher energy rotamers of complex interstellar molecules have been tentatively identified. These higher energy rotamers can be generated through alternative chemical formation pathways leading to the thermally disfavored products. If the transformation between the different rotamers then is hindered because the low temperatures in the interstellar medium prevent the formed higher conformers from overcoming the enegy barriers toward the global minimum configuration, higher energy conformeres can survive and be detected in interstellar objects. For example, the observation of the energetically disfavored trans form of methyl formate toward the Sagittarius B2(N) molecular cloud has been claimed recently.44 In this respect it should be mentioned that, in the case of methyl formate the relative abundance of these features can also depend on the formation pathway of the cluster and/or the desorption mechanism from interstellar ices.

isotopologues, no splitting was detected. The presence of this double structure can be ascribed to a rotation motion of the water molecule around its C2 axis. A similar internal motion of the water moiety was observed also in the analogous rotational spectra of BZN·W.37 Since the motion involves two equivalent hydrogen atoms, an ortho:para (or S:A) spin-statistical weight ratio of 3:1 is expected. Thus, the ground state of this torsional motion is split into two levels, named 0− and 0+ (the strong and the weak respectively, as reported in Figure 3) and two sets of rotational constants were obtained. Both states have been treated independently as asymmetric rotor states regarding the rotational constants whereas the centrifugal distortion constants and the quadrupole coupling constants were fitted to the same values. The transitions were fitted with Pickett’s SPFIT computer program,42 using a standard Hamiltonian that includes centrifugal distortion and nuclear quadrupole coupling terms. Because ACN·W is a near prolate asymmetric top, the Sreduction and the Ir-representation were chosen.43 The obtained spectroscopic constants of the parent species and of the isotopologues, are reported in Table 2. The predicted rotational spectra of ACN·W1 obtained from the experimental constants are reported in Figure 4 at two different temperatures

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STRUCTURE AND DYNAMICS OF ACN·W As a starting point for the least-squares structural refinement performed to reproduce the experimental rotational constants of the four observed water isotopologues (Table 2), we used the ACN monomer geometry taken from an extensive isotopic work on ACN45 and that of water as the standard one. So only the parameters related to the intermolecular interaction were the ab initio (MP2/aug-cc-pVTZ) ones. The fit was performed using Kisiel’s STRFIT program46 and the O8···N4 distance and ∠O8−N4−C3 angle were adjusted. It is known that a shrinking of the hydrogen bond distance takes place when the hydrogen atom involved in the intermolecular bond is substituted with deuterium.47 To take this effect into account, a decrease of the O8···N4 intermolecular distance was introduced in the fit when the rotational constants of deuterated species was considered. The best fit was reached by varying the O···N distance by −0.008 Å, and the values of the adjusted parameters are O8− N4 = 3.169(3) Å and ∠O8−N4−C3 = 79.70(6)°, respectively, and the derived O8−C1 distance is 3.412(4) Å. The 12 rotational constants were reproduced within a few megahertz. Finally, the four sets of rotational constants were used for the determination of Kraitchman’s substitution coordinates48 of the water oxygen and hydrogen atoms. Some information on the structure can be found by analyzing the values of the planar moment of inertia and their sensitivity to isotopic substitution. In particular, the planar moments of inertia along the c axis are negative and close to zero for all isotopologues: Pcc (ACN·W1 (0−)) = −0.1226(2) u Å2, Pcc (ACN·W1 (0+)) = −0.1283(2) u Å2, Pcc (ACN-H18OH (0−)) = −0.1284(5) u Å2, Pcc (ACNH18OH (0+)) = −0.1340(5) u Å2, Pcc (ACN-DOH) = −0.1190(4) u Å2 and Pcc (ACN-DOD) = −0.0739(4) u Å2. The negative value originates from the prevalent effect of the in-plane vibrational modes, although evidence of a large amplitude out-of-plane motion of the water moiety was detected. Because of the small values of the Pcc planar moment of inertia, we imposed a planarity constraint and used Costain’s uncertainties.49 The obtained values are compared to the

Figure 4. Predicted rotational spectra of ACN-W1, with maximum absorption coefficients (α/cm−1), of different transitions at two different temperatures.

(20 and 150 K), which are plausible in different astronomical objects. The lists of the predicted transition frequencies and intensities are available from the authors upon request. The presence of the splitting in the spectrum of the ACN· H18OH isotopologue and its absence in the monodeuterated one are in line with the internal motion described above. On the contrary, in the case of ACN·DOD, we expected to still observe a splitting because of the presence of the two equivalent deuterium atoms. However, such a splitting was not detected probably because, due to a mass effect, it becomes too small to be identified with the resolution of our spectrometer. A similar observation was made for the DOD isotopologue of the BZN·W cluster.37 The observed spectroscopic lines can be used for searches of ACN·W in star-forming regions and other astronomical objects. Moreover, due to the good agreement between the fitted and observed frequencies, also transitions outside the region covered in the present experimental study predicted by the F


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the CH3CN·W adduct, which shows a linear structure.29 Regarding the hydrogen bond parameters, we can observe that they are similar to the ones reported for BZN·W (H···N = 2.257 Å; ∠Hbond···N−C = 89° and ∠OHbond···N = 157°), confirming the similarity of the interaction. As for the internal motion observed, the transition state of the potential energy surface underlying the observed internal rotation was calculated at the same level of theory as the conformational minima, imposing the Cs symmetry to the molecular complex: the oxygen atom of the water was fixed in the ACN plane, and the hydrogen atoms parameters were constrained to be equivalent; all the other parameters were freely optimized. The obtained geometry is reported in Figure 6, and its energy was calculated to be 463 cm−1 above the

corresponding re and r0 values in Table 3, from which it can be seen that the agreement between the two principal coordinates Table 3. Values of the Water Atoms Coordinates (Å) Relating to the Isotopic Substitutions (rs), Obtained from Computational Data (re) and Derived from the Effective Structure (r0) O8 H10 H9

a b a b a b

re

rsa

r0a

2.109 0.750 3.069 0.801 1.911 −0.195

±2.1986(7)b ±0.660(2) ±3.0071(5)c ±0.745(2)c ±1.9430(8) [0]d

2.190(2) 0.702(4) 3.148(2) 0.704(6) 1.9521(5) −0.226(3)

a

The values of the coordinates are calculated from the hypothetically unsplit frequency, obtained by considering the average of the rotational constants of the 0− and 0+ states for the parent species and the H18OH isotopologue. bError expressed in units of the last decimal digit. cDOH as the parent species. dSlightly imaginary value: ±0.415i(4) Å: set to zero.

of the substituted atoms is rather good. The following considerations can be made on the observed small discrepancies. First, because Kraitchman’s method assumes that the nuclei positions do not change upon isotopic substitution, this discrepancy confirms that a structural change takes place when deuterium substitutes a hydrogen atom in the hydrogen bond. Also, the rotational transitions evidence the presence of a large amplitude motion, which could affect differently the rotational constants of the different isotopologues. The derived hydrogen bond parameters for the observed conformation of ACN·W are reported in Figure 5. The nature

Figure 6. Sketch of the transition state for ACN·W, obtained at the MP2/aug-cc-pVTZ level of calculation. The electron density level curves are also shown, in atomic units: purple 0.08−0.04, red 0.04− 0.02, orange 0.02−0.01, and yellow 0.005−0.01 e a0−3 respectively.

minimum. The electron density plot shows that in the transition state the dominating interaction becomes the one between the oxygen and one of the alkene hydrogen atoms. The same ab initio calculation was performed for BZN·W obtaining the internal rotation barrier of 458 cm−1. In the BZN· W paper37 a flexible model analysis was performed in which a model potential function and two structural relaxations functions were adjusted to reproduce the energy difference and the variation of the rotational constants between the observed states. The value of the internal rotation barrier obtained is this analysis was 287(20) cm−1. This barrier is much lower than the calculated one, which was attributed to the fact that a much larger reduced mass is involved when structural relaxations are taken into account. In light of the present theoretical calculations performed in this new work, we suggest that the flexible model analysis might have overestimated the effect of the structural relaxations, although we cannot attempt this type of analysis here because of lack of data regarding the two observed tunneling states.

Figure 5. re and r0 (in bold) geometries, principal inertial axes, and numbering of atoms of ACN·W1.

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of the principal hydrogen bond between the water hydrogen atom and the ACN nitrogen atom is confirmed by a short distance: OH···N = 2.331(3) Å, much shorter than the sum of the van der Waals radii (2.75 Å) of the atoms. Also, the identified secondary interaction between the aliphatic hydrogen atom and the water oxygen atom (Figure 1) can be confirmed by the determined interatomic distance, C−H···O = 2.508(4) Å, which, again, is shorter than the sum of the hydrogen and oxygen van der Waals radii (2.72 Å). Evidently, the presence of this secondary interaction and the formation of a sevenmembered ring is essential in determining the final conformation, which, for example, could not be reached in

CONCLUSIONS We report on the investigation of the rotational spectrum of the 1:1 adduct formed between water and ACN and three of its water isotopologues. The rotational spectra have been recorded and analyzed in the millimeter wave region (59.6−74.4 GHz) and could be used to identify the complex in astronomical observations. From the experimental rotational constants of all the isotopologues the intermolecular interactions have been precisely characterized: they consist of a primary OH···N interaction between the water molecule and the nitrile and a G


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secondary C−H···O link forming a seven-membered ringlike structure. This structure is different from the recently reported CH3CN·W29 where the hydrogen bond is almost linear. The hydrogen bond structure observed for ACN·W is in fact very similar to the one observed for BZN·W and so is the calculated dissociation energy: De = 24.4 and 25.0 kJmol−1 for ACN·W and BZN·W respectively. Evidence of an internal rotation of the water moiety has been observed and the transition state for the motion calculated to be 463 cm−1.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b08426. Fits of the observed lines of ACN·H2O, ACN·D2O, ACN·DOH, and ACN·H18OH using Pickett’s SPFIT computer program and output file of the structural fit (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Wolf Dietrich Geppert. E-mail: [email protected]. *Sonia Melandri. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO). Funding from the COST Action TD1308 for Wolf Geppert is gratefully acknowledged.

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REFERENCES

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Millimeter Wave Spectrum of the Weakly Bound Complex CH2

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