Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide

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Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide– Water Complex by Cryogenic Ion Trap Vibrational Spectroscopy John T Kelly, Harald Knorke, and Knut R. Asmis J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02263 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The Journal of Physical Chemistry Letters

Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide–Water Complex by Cryogenic Ion Trap Vibrational Spectroscopy John T. Kelly, Harald Knorke, and Knut R. Asmis* Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, D-04103 Leipzig, Germany

ABSTRACT

The vibrational spectroscopy of the cyanide–water complex and its fully deuterated isotopologue is studied in the spectral range of 800 to 3800 cm-1. Infrared/infrared double-resonance population labeling spectroscopy of the cryogenically cooled, messenger-tagged complexes isolates the spectral signature of the two quasi-isoenergetic, singly hydrogen-bonded isomers HOHNC¯ and HOHCN¯. The infrared photodissociation spectra are assigned based on a comparison to simulated anharmonic spectra. Infrared multiple photon dissociation spectra in the temperature range from 6 to 300 K confirm the stability of the two isomers at lower temperatures and provide evidence for a considerably more dynamic structure involving doubly-hydrogen-bonded configurations at higher internal energies. The observed red shifts ΔνOH of the hydrogen-bonded O-H stretches,

671 cm-1 (HOHNCˉ) and 812 cm-1 (HOHCNˉ), confirm the universal

correlation of ΔνOH with the corresponding proton affinities.

ACS Paragon Plus Environment

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A molecular level characterization of how anions are hydrated is crucial for the fundamental understanding of chemical reaction dynamics in aqueous solution as well as at the air/water interface.1-4 Valuable information on the anion–water interaction can be extracted from highly selective and sensitive spectroscopic experiments on gas phase clusters.5-8 In particular, vibrational spectroscopy on microhydrated halide ions, X¯(H2O)n, have served as ideal model systems to uncover the paradigms of anion solvation.7, 9-10 Interestingly, hydration of the diatomic cyanide anion, a prototypical pseudohalogen exhibiting an ellipsoidal charge distribution, which is best known for its natural occurrence in microorganisms and the interstellar medium, its applications in coordination chemistry, its wide spread industrial use and its high toxicity, has not been fully characterized.11 Only recently, Wang and coworkers investigated the [CN¯, H2O] complex by anion photoelectron spectroscopy and compared their results to those of [Cl¯, H2O] considering the similar ionic radii, polarizability, and electron affinity.12-13 High level quantum chemical computations identify two nearly isoenergetic isomers (ΔE0 < 1 kJ/mol) that differ in which atom accepts the hydrogen bond, i.e. HOHNC¯ and HOHCN¯.12, 14 Interestingly, only a single isomer, HOHNC¯, was assigned experimentally based on the measured vertical detachment energy. Here, we characterize, for the first time, the gas phase vibrational spectroscopy of the cyanide–water complex and its fully deuterated analog using cryogenic ion trap vibrational spectroscopy15-16 combined with an isomer-specific detection scheme17-18 and temperature dependent measurements.19-22 This study sheds new light on the effect of hydrogen bonding on the properties of this ubiquitous diatomic anion, in particular, the relative stability of the isomeric forms as well as the correlation between spectral frequency shift, hydrogen bond strength, vibrational anharmonicity and proton affinity.


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Figure 1. IRPD spectrum (black trace) of the H2-tagged cyanide–water complex [CN¯, H2O] measured at an ion trap temperature of 12 K compared to two IR2MS2 spectra probed at 2896 cm1

(a6, blue trace) and 3034 cm-1 (a5, red trace). The spectra are assigned to the two isomers shown

(H: white, C: grey, N: blue, O: red), containing a water molecule singly hydrogen-bonded to either the C- (HOHCN¯, blue trace) or the N-atom (HOHNC¯, red trace). See Table 1 for band positions and assignments.

Figure 1 shows the IRPD spectrum of H2-tagged cyanide–water complex in the O-H stretching region from 2750 to 3800 cm-1. Six bands, labeled a1 to a6, are observed and their


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positions are listed in Table 1. Naively, only two IRPD bands are expected. For a singly hydrogenbonded complex these correspond to a free O-H oscillator with low IR activity, expected around 3700 cm-1, and a hydrogen-bonded O-H oscillator, typically found below 3400 cm-1, with a redshift that is correlated to the hydrogen bond strength and a high IR activity due to charge transfer along the proton transfer coordinate.23-24 The observation of more than two features thus suggests that anharmonic effects and/or the presence of multiple isomers need to be considered in assigning the IRPD spectrum. To investigate the isomeric contributions to the IRPD spectrum we performed isomerselective IR2MS2 measurements.18, 25-26 IR2MS2 spectra (see Figure 1) probed in the hydrogenbonded O-H oscillator region at 2896 cm-1 (a6, blue trace) and 3034 cm-1 (a5, red trace) reveal the characteristic IR signatures of two stable isomers with hydrogen-bonded O-H stretch fundamentals at 2895 cm-1 (a6') and 3035 cm-1 (a5''), respectively. The two IR2MS2 spectra are similar in form but shifted in energy relative to each other, in line with the expected presence of two nearly isoenergetic, singly hydrogen-bonded isomers that differ in H2O binding either to the C-atom or the N-atom. Interestingly, the intensity ratio of the two O-H stretch fundamentals a5 and a6 of roughly 2:1 suggests that the isomer with the less red-shifted fundamental is more abundantly present and hence slightly more stable. This observation is in agreement with the results of high level electronic structure calculations, which predict a small preference for water binding to the Natom of cyanide as a result of the tighter N-atom lone pair orbital and hence a slightly stronger hydrogen bond.12, 14 Our calculations (vide infra) confirm this interesting observation (see Table S1 in the supporting information). The calculated hydrogen bond length in HOHNC¯ (1.780 Å) is shorter than in HOHCN¯ (1.891 Å), suggesting a stronger hydrogen bond. However, the corresponding hydrogen-bonded O-H bond also remains shorter (0.995 Å vs. 1.000 Å) and


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therefore a less pronounced red-shift, due to less charge transfer and hence a weaker perturbation of the water molecule in HOHNC¯. Hence, we assign the blue and red traces in Figure 1 to the IR spectra of the HOHNC¯ and HOHCN¯ isomers, respectively, and attribute bands a2'', a3'', and a4 ' to non-fundamental transitions. Note, the free O-H stretches (a1) are too weak in intensity to be clearly resolved in the two-color IR2MS2 spectra.

Figure 2. Comparison of simulated VPT2/MP2/aug-cc-pVTZ anharmonic (a,d) and experimental IRPD (b,c) spectra of the all-H (a,b) and all-D (c,d) isotopologues of cyanide–water complexes in the spectral range of 800 – 4000 cm-1. The simulated spectra represent the weighted sum (2:1) of the anharmonic spectra of the HOHNC¯ and HOHCN¯ isomers (see SI for the individual


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harmonic and anharmonic spectra), which were convoluted with a Gaussian line function (15 cm1

full width at half maximum). The IRPD spectra were measured of by photodissociation of

messenger-tagged complexes, H2 for [CN¯,H2O] and D2 for [CN¯,D2O]. For better comparability the spectra are plotted against opposing ordinates and the bottom abscissa (for the all-D isotopologue) has been scaled by 1.36 (see text). See Table 1 for band positions and assignments.


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Table 1. Experimental band positions (in cm-1, labels in parentheses), H/D isotopologue vibrational frequency ratios, VPT2/MP2/augcc-pVTZ anharmonic vibrational frequencies (in cm-1, anharmonic/harmonic frequency ratio in parentheses) and band assignments of the features observed in the vibrational spectra of the cyanide–water complexes shown in Figures 1 and 2.

Experiment

Theory

[CN¯,H2O,H2] IRPD IR2MS2 H2-tag 2896 cm-1 3705 (a1)

IR2MS2 3034 cm-1

[CN¯,D2O,D2] IRPD H/DD2-tag ratio 2731 (b1) 1.36

3349 (a2)

3347 (a''2)

2507 (b2)

1.34

3284 (a3)

3285 (a''3)

2457 (b3)

1.34

2400 (b4)

1.30

2278 (b5)

1.33

3126 (a4)

3128 (a'4)

3034 (a5) 2896 (a6)

3036 (a''5) 2895 (a'6)

Assignment

HOHˑˑˑCN¯

DODˑˑˑCN¯

HOHˑˑˑNC¯

DODˑˑˑNC¯

VPT2

VPT2

VPT2

VPT2

3712 (0.96)

2735 (0.97)

3720 (0.95)

2742 (0.97)

free O-H stretch

2403

3282

2417

H-O-H bend overtone

3195

2442

H-bonded O-H stretch + H-bond stretch

2948 (0.91)

2206 (0.94)

1970 (0.99)

1969 (0.98)

C-N stretch

1672 (0.98)

1225 (0.99)

H-O-H bend

3013

H-bonded O-H stretch + H-bond stretch

2192 (b6)

1.32

2785 (0.89)

2103 (0.92)

2058 (a7)

2059 (b7)

1.00

1976 (0.99)

1976 (0,99)

1672 (a8)

1227 (b8)

1.36

1572 (a9)

1157 (b9)

1.36

1618 (0.96)

1194 (0.97)

1557 (a10)

1147 (b10)

1.36 870

631

H-bonded O-H stretch H-bonded O-H stretch

H-O-H bend 1601

1182

out-of-plane bend overtone

846

615

out-of-plane bend


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The (single-color) IRPD spectrum of the H2-tagged cyanide–water complex is shown over a larger spectral range (800-3800 cm-1) in Figure 2, together with the spectrum of the D 2-tagged fully deuterated isotopologue. To aid in the spectral assignment we performed electronic structure calculations (see Figures S1 and S2, as well as Tables S2 to S6). The corresponding simulated anharmonic spectra (VPT2/MP2/aug-cc-pVTZ) assuming an isomeric ratio of 2:1 in favor of the HOHNC¯ isomer are also shown in Figure 2. Band positions, H/D ratios for the vibrational frequencies of the two isotopologues, anharmonic frequencies and band assignments are summarized in Table 1. For better comparability, a scaling factor (1/1.36) is used for the alignment of the two wavenumber scales in Figure 2, which brings the free O-H stretching (a1,b1) as well as the water bending fundamentals (a8/b8 and a9/b9), one pair for each isomer, of the two isotopologues into alignment.22 Bands a7 (2058 cm-1) and b7 (2059 cm-1) clearly correspond to the CN stretch fundamental, because its absolute position remains nearly unchanged upon isotope exchange (H/D ratio: 1.00). It appears blue-shifted with respect to the vibrational frequency of bare CN– (2035 cm-1)27 as a result of the removal of electron density from an antibonding orbital upon water complexation.14 Comparison of the experimental spectra with the simulated anharmonic frequencies identify the remaining bands as the water bend overtone (a2/b2), combination bands of the hydrogen bonded O-H stretch with the intramolecular (IM) stretch for both isomers (a3/b3 and a4/b4), and the intermolecular out-of-plane (oop) bend overtone (a10/b10). In summary, the simulated anharmonic spectra qualitatively recover the relative energies and intensities of all of the significant features. Anharmonicities are predicted to be small, except for transitions involving the hydrogen-bonded O-H stretching modes (see Table 1). Upon deuteration these anharmonicities are reduced by roughly 30%, e.g. the anharmonic/harmonic frequency ratio increases from 0.89 to 0.92 and from 0.91 to 0.94 for the HOH/DODCN¯ and


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HOH/DODNC¯ isomers, respectively. The discrepancy between experimental and predicted vibrational frequencies is generally largest for the most anharmonic transitions, suggesting that higher dimensional approaches are necessary to obtain more quantitative agreement whenever motion along the proton transfer coordinate is involved.

Figure 3. Infrared multiphoton dissociation (IRMPD) spectra of the cyanide–water complex [CN¯,H2O] in the O-H stretching region measured at ion-trap temperatures ranging from 6 K to 300 K (b-h) and IRPD spectrum of the messenger-tagged complex [CN¯,H2O,H2] at 12 K (a). See Table 1 for band assignments.


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Temperature-dependent infrared multiphoton dissociation (IRMPD) spectra of cyanide– water complex [CN¯,H2O], measured from 6 K to 300 K, are shown in Figure 3. This technique has been successfully employed previously to investigate the temperature stability of individual isomers20, 22 as well as the thermodynamics of isomer interconversion21 in hydrogen-bonded ions. The IRMPD spectrum measured at 6 K looks quite similar in form to the IRPD spectrum of the H2-tagged complex measured at 12 K, but some of the lower energy IRMPD features show a remarkable red-shift (with respect to the bands in the IRPD spectrum) of up to 24 cm-1 and the relative intensities differ somewhat too. Since our calculations provide no evidence for such a characteristic messenger effect, it is more likely that these effects are the result of the multiple photon absorption mechanism. Note, at least two IR photons are needed in the spectral region of interest to overcome the water binding energy of 53 kJ/mol (4442 cm-1).28 Up to a temperature of about 150 K the IRMPD spectra show the typical signature of thermal broadening, due to population of higher rotational and low-frequency vibrational levels of a stable isomer, similar to what is observed in the IRMPD spectra of NO3¯(H2O) and H+(H2O)5.20, 22

Above 150 K, however, substantial IR intensity is transferred from the spectral region below

3100 cm-1 to the region around 3400 cm-1, suggesting a weakening of the hydrogen bonds. At 300 K a quasi-continuous absorption ranging from 2800 to 3800 cm-1 with broad maxima around 3100 and 3400 cm-1 as well additional activity in the free O-H stretching region around 3700 cm-1 is observed. This can be rationalized assuming a dynamically favored, doubly hydrogen-bonded configuration much like in Cl¯(H2O).12 While at low temperatures the cluster adopts a singlyhydrogen bonded structure close to the global minimum, at elevated temperatures the water rocking mode is activated, which leads to a much more dynamic structure with substantial probability close to the doubly hydrogen-bonded configuration, which represents a first order


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transition state on the potential energy surface. 12 Compared to the singly hydrogen-bonded minimum energy structure, it contains two considerably weaker hydrogen bonds, explaining why the IR activity shifts to higher photon energies with increasing temperature (see Table S8 in the SI). Finally, we discuss the red-shift ΔνOH of the hydrogen-bond O-H stretching frequencies in the complex HOHXˉ in a thermodynamic context. As noted previously by Robertson and Johnson, the extent of ΔνOH, defined relative to the frequency of a free uncoupled O-H oscillator, given by the centroid of the symmetric and antisymmetric OH stretches in bare water (3707 cm-1), correlates with the proton affinity (PA) of the anion Xˉ.7 The ΔνOH values for HOHNCˉ and HOHCNˉ determined in the present work are 671 cm-1 and 812 cm-1, respectively, and lie inbetween the observed red-shifts for HOHClˉ (561 cm-1)23

and HOHO2ˉ (1082 cm-1)29,

suggesting PAs of CNˉ lie in-between 1395 kJ/mol (Clˉ)30 and 1477 kJ/mol (O2ˉ)31 Indeed the PAs (at 0 K) of CNˉ, determined from the thermodynamic cycle32 described in the SI, are 1400 kJ/mol (CNˉ + H+  HNC) and 1462 kJ/mol (CNˉ + H+  HCN), confirming the validity of this universal relationship (see Figure S3). In summary, the infrared photodissociation spectrum of the cyanide–water complex measured at cryogenic temperatures contains clear evidence for the presence two stable, singly hydrogen-bonded isomers, whose spectroscopic signatures can be isolated using isomer-specific IR2MS2 spectroscopy and completely assigned when anharmonic effects are considered. The HOHNCˉ isomer, which is slightly more stable than HOHCNˉ, is more abundantly present, but characterized by a slightly less red-shifted hydrogen-bonded O-H stretching frequency as a results of the tighter N-atom lone pair orbital. The present data qualitatively confirms the universal


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relationship between the red-shift of the hydrogen-bonded O-H stretches and the anion proton affinities.24 Temperature–dependent IRMPD spectra provide evidence for the increasingly dynamic behavior at elevated temperatures (>150 K), which is due to activation of the water rocking motion and leads to considerable contributions to the IR spectrum from structures containing a doubly hydrogen-bonded water molecule. Studies on how these binding motifs evolve as the degree of microhydration is increased are currently in process.

EXPERIMENTAL METHODS IRPD experiments were performed using a 6 K ion-trap triple mass spectrometer described previously.33-34 In brief, [CNˉ,H2O/D2O] anions were generated in a nano-electrospray source from a 10 mM solution of KCN in a 2:1 methanol to water solution, thermalized at room temperature in a gas-filled radio-frequency (RF) ion-guide, mass-selected using a quadrupole mass filter and focused in a RF ring-electrode ion-trap. The trap, filled with pure He (IRMPD experiments), H2 or D2 (IRPD experiments) buffer gas, is cooled with a closed-cycle He cryostat and held at temperatures between 6 K and 300 K. Many collisions of the trapped ions with the buffer gas provide gentle cooling of the internal degrees of freedom close to the ambient temperature. At sufficiently low ion-trap temperatures, ion-messenger complexes are formed via three body collisions.16, 35 Every 100 ms all ions are extracted and focused both temporally and spatially into the center of the extraction region of an orthogonally mounted reflectron time-of-flight (TOF) tandem photofragmentation mass spectrometer. Here, the ion packet is irradiated with tunable infrared radiation supplied by an optical parametric oscillator/amplifier (LaserVision: OPO/OPA/AgGaSe2) laser system.36 All parent and photofragment ions are then accelerated


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towards an MCP detector. An IRPD spectrum is measured by continuously scanning the laser wavelength, which is monitored online using a HighFinesse WS6-600 wavelength meter, with a scan speed such that an averaged TOF mass spectrum (over 100 laser shots) is obtained every 2 cm-1. Typically, at least three scans are measured, averaged and the photodissociation cross section IRPD is determined as described previously.33-34 Double-resonance IR2MS2 spectra using the iondip technique are obtained by employing two tunable IR lasers in a pump-probe fashion22, 37 and tandem mass-selection stages.18 The monitored IR2MS2 ion signals are corrected by an impact energy dependent scaling factor to account for different detection efficiencies. THEORETICAL METHODS Ab initio electronic structure computations were carried out using the Gaussian09 D.01 software package.38 Optimized geometries and harmonic frequencies were obtained using the MP2 method39 in combination with the aug-cc-pVTZ basis set.40 Anharmonic vibrational frequencies were calculated using second-order vibrational perturbation theory (VPT2).41 Geometries and frequencies are reported in the Supporting Information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +49 (0) 34197 36421 (K.R.A.) ORCID: 0000-00016297-5856

ACKNOWLEDGMENT


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This work has been supported by the German Research Foundation (DFG) as part of the individual research grant AS133/3-1 "Spectroscopic Characterization of Salt Dissolution in Microhydrated Cluster Ions and at the Water / Vapor Interface”. We are grateful to Alexandra Giermann (Universität Leipzig) for her instructions, guidance, and safety protocols for handling cyanide solutions. K.R.A. acknowledges instrumental support from the Fritz-Haber-Institute of the MaxPlanck-Society. ASSOCIATED CONTENT Supporting Information Available: Cartesian coordinates, vibrational frequencies, and plot of proton affinity versus red-shifted O-H stretch (OH) are included in the Supporting Information.

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Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide

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