Stability of the Ground and Low-Lying Vibrational States of the

Letter pubs.acs.org/JPCL

Stability of the Ground and Low-Lying Vibrational States of the Ammonium Radical John D. Savee,† Jennifer E. Mann,‡ and Robert E. Continetti* Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0340, United States ABSTRACT: The tetrahedral ammonium radicals NH4 and ND4 are exotic neutral molecules that can be produced by charge exchange (CE) neutralization of the ammonium cation. In the present study, the stability of these radicals was measured by translational spectroscopy on the NH3/ND3 + H/D fragments resulting from CE between NH4+/ND4+ and cesium. The vibrationally resolved spectrum reveals the energy of the 3s(2A1) ground state of the ammonium radical relative to ground-state NH3 + H dissociation products as well as the energies for the singly excited ν1 and ν2 modes of the NH4 isotopologues. Using these values, a lower limit for the heat of formation of NH4, ΔHf,0K° = (189.7 ± 0.4) kJ/mol has been determined, as well as an adiabatic ionization energy of (4.62 ± 0.01) eV for NH4, in good agreement with recent predictions.

SECTION: Spectroscopy, Photochemistry, and Excited States he NH3 + H ↔ NH2 + H2 reaction is a prototypical fiveatom system with a slow forward rate at room temperature resulting from a considerable dynamical barrier on the underlying ground 3s(2A1) NH4 potential energy surface (PES). A potential well deep enough to support at least a few metastable vibrational states of the ammonium radical (NH4) on the 3s PES is known to exist, although shallow wells are accompanied by significant anharmonic effects that are difficult to characterize computationally. Experimentally, it has been difficult to isolate NH4 due to an extremely short ground-state lifetime (∼10 ps).1,2 The perdeuterated ammonium radical, ND4, has a significantly longer lifetime (∼30 μs),3,4 suggesting that tunneling through the barrier separating NH4 from NH3 + H is the principal mechanism by which the ground state decays. The longer lifetime of ND4 has made it an inviting alternative for use in spectroscopic probes of the ammonium radical. The Schüler emission band observed in ammonia discharges5 was interpreted by Herzberg to arise from a transition in the NH4 radical,6 but the diffuse and weak transitions allowed only speculative assignment of the spectroscopic origin. Using a series of rotationally resolved absorption and emission measurements of the ND4 Schüler band, Watson and coworkers were able to definitively assign the origin as the 3p(2T2) ↔ 3s(2A1) transition of ND4.1,4,6−8 Although never directly confirmed by rotational analysis, the Schüler emission bands of NH4 are generally assumed to occur via the analogous transition. Charge exchange (CE) between the stable gas-phase ammonium cation (NH4+) and an electron-donating species is a well-known method for producing the corresponding shortlived neutral radical. Porter and co-workers examined spatial profiles of recoiling products from CE between NH4+/ND4+

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and K or Na atoms, providing a measure of the thermodynamic stability of ammonium radical isotopologues and the barrier separating NH4 from NH3 + H.3,9 Ketterle et al.10 used CE with cesium (Cs) and observed Schüler band emission from both NH4 and ND4, indicating that at least some neutral ammonium radicals were formed in the 3p(ν = 0) state. This study also showed that the lifetime of the 3p state of ND4 (∼4 ns) was significantly longer than that of NH4, suggesting that the 3p state decays via nonradiative processes in addition to Schüler emission. The present experiments use translational spectroscopy and time- and position-sensitive neutral particle detection techniques to examine the total center-of-mass kinetic energy release (KER) for the NH3 + H and ND3 + D product channels resulting from CE of fast (16 keV) NH4+/ND4+ ion beams with Cs. The coincidence methods employed here are capable of measuring the total KER imparted to NH3 + H or ND3 + D products on a per-event basis, and these events can be histogrammed into a P(KER) probability distribution that provides a unique energetic probe of the transient neutral precursor (i.e., NH4/ND4). A ∼0.03 eV fwhm upper limit to the KER resolution was determined for the conditions used in the present experiments with an associated accuracy of 0.005 eV.11 As discussed in a recent review of this technique and illustrated in Figure 1,11 the shape of spectral features in P(KER) distributions can be indicative of the mechanism(s) for decay of the initially excited neutral state to the observed products. When at least one diatomic or polyatomic fragment is Received: September 4, 2013 Accepted: October 15, 2013 Published: October 15, 2013 3683

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Figure 1. Energetics for the CE of NH4+ with Cs.

produced, broad P(KER) features can be characteristic of initial neutral states that are either directly repulsive or predissociated by a repulsive electronic state, whereas sharp spectral features suggest predissociation of a quasi-bound state (e.g., tunneling through a barrier). If appropriate assumptions can be made about the final state distributions in the products, sharp features in the KER spectra and the high-KER onset for broad features can provide direct measures of where initial states of the neutralized molecule lie energetically relative to product asymptotes. Populations of the initially excited NH4 states produced by CE are challenging to predict a priori, although transition probabilities can be rationalized in part based on how far neutral states lie energetically from resonant excitation as well as the Franck−Condon (FC) overlap with the parent cation.11−14 Resonant CE with Cs (indicated by the dashed orange arrow in Figure 1) energetically favors states ∼0.8 eV above the 3s(ν = 0) state of NH4, which is farther from the anticipated location of the 3p state (∼1.8 eV above 3s(ν = 0)15−19 or ∼1 eV above resonant excitation), thus slightly favoring production in the 3s state on energetic grounds. The tetrahedral (Td) ground electronic state of NH4+ is anticipated to have reasonable FC overlap with the 3s(ν = 0) state of NH4.20−23 Early ab initio calculations suggested weak Jahn− Teller distortions from Td symmetry for the triply degenerate 3p state of NH4,17,24 supporting that this state will also have favorable FC overlap with NH4+. However, recent investigations of the triply degenerate ground electronic state of CH4+ found significant distortions from Td symmetry,25 and if this is also true of the 3p state of NH4, it would cause weaker FC overlap with the cation and a lowered transition probability for electron capture. Na+ is the isoelectronic atomic analogue of NH4+, and cross sections for CE between Na+ and Cs at high (keV) incident ion beam energies have been calculated by de Andres et al.,26 showing that electron capture into the 32P states of neutral sodium (analogous to the 3p state of NH4) is favored over capture into the 32S state (analogous to the 3s state of NH4) by a factor of ∼2.75. The Na+ CE calculations also suggest that capture of an electron into the higher-lying 42D states is preferred over the 32P states by an additional factor of ∼2.5. While previous experiments have confirmed population of the 3p(ν = 0) state of NH4 and ND4 via CE with Cs through observation of Schüler emission,10 we cannot rule

Figure 2. P(KER) distributions obtained from CE between (a) NH4+ or (b) ND4+ and Cs. The thinner black line is the data scaled by an arbitrary factor to magnify low-intensity features. The orange combs illustrate various combinations of excitation in the ν1 symmetric stretch mode of NH3/ND3 products with ν2 = 0 or 1. The Schüler emission bands reported in ref 1 are included as green lines originating at 3pmax (see text).

out that the lower 3s PES and potentially other higher-lying (e.g., 3d) states are also initially populated. P(KER) distributions measured in the present work for NH3 + H products are presented in Figure 2a. No significant NH2 + H2 product channel was observed. At low KER, the NH3 + H spectrum is characterized by a sharp feature near 0.13 eV accompanied by three peaks of lesser intensity at 0.34, 0.44, and 0.54 eV (the latter being weak, marked by an asterisk in the figure). In the case of ND4 → ND3 + D (Figure 2b), three peaks at 0.19, 0.32, and 0.43 eV are observed. At high KER in the NH4 spectrum, a series of four doublets originating at 2.00 eV (labeled 3pmax) and spaced toward lower KER by 0.41 eV (∼3310 cm−1) and split by 0.12 eV (∼970 cm−1) are also observed. The corresponding doublet features in the ND3 + D P(KER) spectrum (Figure 2b) are barely resolved but exhibit spacings of 0.30 (∼2420 cm−1) and 0.09 eV (∼730 cm−1). This isotope effect is consistent with production of NH3/ND3 with various degrees of excitation in the symmetric stretch (ν1 = 3337/2420 cm−1) and umbrella modes (ν2 = 950/748 cm−1).27 The 3s(ν = 0) state of NH4 is expected to lie higher than the NH3(X̃ 1A1) + H(2S) dissociation limit by ∼0.1−0.2 eV.3,28,29 The sharp P(KER) feature at 0.13 eV is consistent with tunneling-induced predissociation of the 3s(ν = 0) state, assigned in part by the aforementioned energetic predictions and also from assignment of this feature as resulting from dissociation of the lower state of the principal Schüler emission band (discussed below). The similar equilibrium N−H bond lengths of NH4 3s(ν = 0) (1.0365 Å)20 and that in free ammonia (1.012 Å)30 suggest that this predissociation will favor vibrationless NH3 + H products. In the absence of significant rotational excitation in the vibrationless NH3 product, the 3s(ν = 0) P(KER) feature provides a direct measurement of the 3684

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measurements place their energies relative to NH3 + H (0.44 and 0.34 eV, respectively). The ND4 3s(ν = 0) state is expected to be nearly isoenergetic with the ND3 + D product limit.3 In the present experiments, the 0.00−0.05 eV region of the P(KER) spectra was found to be dominated by false coincidence events obscuring the 3s(ν = 0) peak, consistent with the formation of ND4 in this state that is stable during the ∼3 μs flight time from the CE cell to the multiparticle detector. By constructing a comb corresponding to the principal (∼14810 cm−1) and weaker (∼13680 and 12850 cm−1) Schüler bands of ND41 and aligning the weaker emission bands attributed to single 3s ν1 (1960 cm−1) and ν2 (1134 cm−1) excitations to the P(KER) features at 0.19 and 0.32 eV (see Figure 2), the 3s(ν = 0) state is anticipated to lie 0.08 eV above the ND3 + D limit. The pronounced peak at 0.43 eV (∼0.35 eV or 2820 cm−1 above the 3s(ν = 0) feature) in the ND4 → ND3 + D P(KER) spectrum (Figure 2b) is not consistent with any ab-initiodetermined ND4 fundamental vibrational modes17,36 and is tentatively assigned to predissociation of a quasi-bound ν1 + ν2 combination band on the ND4 3s PES (3094 cm−1 from frequencies derived from the Schüler bands). In addition, the feature at 0.54 eV in the NH4 → NH3 + H P(KER) spectrum (∼0.41 eV or 3250 cm−1 above the assigned 3s(ν = 0) peak, marked by an asterisk) is not consistent with the analogous ν1 + ν2 frequency derived from the Schüler bands (4133 cm−1) and is tentatively assigned to predissociation of the 2ν2 mode of the 3s state of NH4 (3162 cm−1 derived from the Schüler bands). Schüler band emission corresponding to radiative decay from 3p(ν = 0) to these tentatively assigned combination or overtone bands of ND4 and NH4 has not been reported, although it is likely to be very weak in comparison to the principal bands. It is also possible that the resolved 3s vibrational features in the P(KER) spectra are in part populated directly by CE. Weak signal above 3pmax in the P(KER) spectra presented in Figure 2 suggests that initial states of NH4 and ND4 lying above the 3p state are also populated by CE. Although the signal-to-noise in this region does not allow quantitative energetics to be determined for these higher-lying states of NH4, their population provides additional potential routes to formation of the observed predissociated states of NH4 and ND4. The mechanism by which these quasi-bound 3s states are populated will not affect the KER associated with tunneling-induced predissociation, allowing assignment of the energetics relative to NH3 + H or ND3 + D. By reinvestigating CE between NH4+/ND4+ and Cs with state-of-the-art methods in translational spectroscopy, energetics of the transient NH4 and ND4 species have been revealed in unprecedented detail. Several important properties for the NH4 free radical have been obtained, including a new determination of the AIE and the 0 K heat of formation. Although it is generally assumed to have the same origins as in ND4, observation of vibrationally resolved levels of the 3s(2A1) ground state of NH4 provides further confirmation that the Schüler emission arises from a vibrationless 3p(2T2) → 3s(2A1) transition in NH 4 . These results are of fundamental spectroscopic interest due to the complex electronic structure of the NH4 radical and will provide a critical test of future studies of the NH4 PES with particular relevance to the NH3 + H hydrogen exchange reaction where quantum effects from the well associated with NH4 may play a significant role.29

reaction enthalpy between this state of NH4 and the NH3(X̃ 1A1) + H(2S) limit, as depicted in Figure 1. Because 0 K heats of formation of the dissociation products are wellknown (ΔHf,0K = (−38.907 ± 0.4) and (216.035 ± 0.006) kJ/ mol for NH3 and H, respectively),31 we can infer a lower limit to the gas-phase value of ΔHf,0K°(NH4) = (189.7 ± 0.4) kJ/ mol.

Figure 3. Thermodynamic cycle used to evaluate the AIE of NH4.

Using a thermodynamic cycle like that illustrated in Figure 3 involving the reaction enthalpy (KER = 0.13 eV), the ionization energy of the H-atom (IE(H) = 13.60 eV),27 and the proton affinity of ammonia (PA(NH3) = 853.6 kJ/mol),32 an adiabatic ionization energy (AIE) of (4.62 ± 0.01) eV is obtained for NH4. This value is lower than the value of (4.73 ± 0.06) eV obtained from the low-resolution CE experiments by Gellene et al.3 but in agreement with the estimated value of 4.62 eV from experiments by Fuke et al.33 and recent computed values.18,34,35 It should be noted that this value is also slightly lower than the high-precision AIE = (4.64826 ± 0.00019) eV for ND4 measured by Signorell et al.,21 consistent with the isotope effect on the AIE for species like CH4 and H2O.27 Ab initio calculations predict that the 3p state of NH4 lies 1.7−1.9 eV above the 3s(ν = 0) state.15−19 The spacing between the assigned 3s(ν = 0) peak and the highest-energy resolved feature (3pmax) is 1.87 eV (∼15083 cm−1), closely matching the principal Schüler band (∼15067 cm−1).1,5 The 3p 2 T2 state of NH4 does not adiabatically correlate with ground electronic state NH3 + H products,19 suggesting that predissociation of the 3p state via internal conversion to the 3s ground state of NH4 leads to NH3 in a range of vibrational states. The highest-energy resolved KER feature, 3pmax, is consistent with production of NH3 in the ground vibrational state. The orange comb in Figure 2a shows the assignments of the series of doublets to excitation of the symmetric stretch (ν1) mode of NH3, with the doublet structure arising from combination bands involving one quantum of excitation in the umbrella (ν2) mode. The green comb in Figure 2a is constructed from the principal NH4 Schüler emission band, and the associated weaker Schüler bands at lower energies (∼13486 and 12515 cm−1) are attributed to transitions from 3p(ν = 0) to vibrationally excited 3s states.1,5 The observation of features corresponding to these emission bands in the NH3 + H P(KER) spectrum is consistent with the Schüler bands arising from a 3p → 3s transition, confirming the assignment of the 3pmax feature to predissociation of the NH4 3p(ν = 0) state to NH3(ν1 = 0, ν2 = 0) + H products. The lower-energy Schüler bands have been previously assigned to fluorescence to lower 3s states with single quanta of excitation in the a1 (ν1 = 2552 cm−1) and e (ν2 = 1581 cm−1) modes,1 and the present 3685

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in the 3p2T2 State of the Ammonium Radical. Chem. Phys. 1988, 123, 73−83. (18) Ortiz, J. V.; Martin, I.; Velasco, A. M.; Lavin, C. Ground and Excited States of NH4: Electron Propagator and Quantum Defect Analysis. J. Chem. Phys. 2004, 120, 7949−7954. (19) Park, J. K. Avoided Curve Crossing for the Dissociation of the Rydberg NH4 Radical into (NH3+H). J. Chem. Phys. 1997, 107, 6795− 6803. (20) Sattelmeyer, K. W.; Schaefer, H. F.; Stanton, J. F. The Equilibrium Structure of the Ammonium Radical Rydberg Ground State. J. Chem. Phys. 2001, 114, 9863−9865. (21) Signorell, R.; Palm, H.; Merkt, F. Structure of the Ammonium Radical from a Rotationally Resolved Photoelectron Spectrum. J. Chem. Phys. 1997, 106, 6523−6533. (22) Martin, J. M. L.; Lee, T. J. Accurate Ab Initio Quartic Force Field and Vibrational Frequencies of the NH4+ Ion and Its Deuterated Forms. Chem. Phys. Lett. 1996, 258, 129−135. (23) Crofton, M. W.; Oka, T. Observation of Forbidden Transitions of Ammonium Ion (NH4+) ν3 Band and Determination of GroundState Rotational-Constants  Observation of ν3 Band Allowed Transitions of ND4+. J. Chem. Phys. 1987, 86, 5983−5988. (24) Havriliak, S.; King, H. F. Rydberg Radicals. 1. Frozen-Core Model for Rydberg Levels of the Ammonium Radical. J. Am. Chem. Soc. 1983, 105, 4−12. (25) Wörner, H. J.; Qian, X.; Merkt, F. Jahn−Teller Effect in Tetrahedral Symmetry: Large-Amplitude Tunneling Motion and Rovibronic Structure of CH4+ and CD4+. J. Chem. Phys. 2007, 126, 144305. (26) de Andres, J.; Sabido, M.; Aricha, M. E.; Alberti, M.; Lucas, J. M.; Gadea, F. X.; Aguilar, A. An Experimental and Theoretical Study of Electronic Excitation and Charge Transfer Processes in Collisions between Cs(62S1/2) Atoms and Na+(1S0) Ions in the 0.30−4.00 keV Energy Range. Chem. Phys. 2002, 281, 33−47. (27) Lias, S. G.; Liebman, J. F. NIST Chemistry Webbook; National Institute of Standards and Technology: Gaithersburg, MD, 2013. (28) Boldyrev, A. I.; Simons, J. Theoretical Search for Large Rydberg Molecules  NH3CH3, NH2(CH3)2, NH(CH3)3, and N(CH3)4. J. Chem. Phys. 1992, 97, 6621−6627. (29) Moyano, G. E.; Collins, M. A. Interpolated Potential Energy Surface for Abstraction and Exchange Reactions of NH3+H and Deuterated Analogues. Theor. Chem. Acc. 2005, 113, 225−232. (30) Herzberg, G. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: New York, 1966. (31) Chase, M. W.; Davies, C. A.; Downey, J. R.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. NIST JANAF Thermochemical Tables; National Institute of Standards and Technology: Gaithersburg, MD, 2013. (32) Hunter, E. P. L.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27, 413−656. (33) Fuke, K.; Takasu, R.; Misaizu, F. Photoionization of Hypervalent Molecular Clusters  Electronic-Structure and Stability of NH4(NH3)N. Chem. Phys. Lett. 1994, 229, 597−603. (34) Chen, F. W.; Davidson, E. R. Electronic, Structural, and Hyperfine Interaction Investigations on Rydberg Molecules: NH4, OH3, and FH2. J. Phys. Chem. A 2001, 105, 10915−10921. (35) Velasco, A. M.; Lavin, C.; Martin, I.; Melin, J.; Ortiz, J. V. Partial Photoionization Cross Sections of NH4 and H3O Rydberg Radicals. J. Chem. Phys. 2009, 131, 024104. (36) Kaspar, J.; Smith, V. H.; McMaster, B. N. The Stability and Spectra of Isotopic Forms of the Ammonium Radical  An Ab Initio CI Study of the Dissociation Barrier. Chem. Phys. 1985, 96, 81−95.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

J.D.S.: Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551. ‡ J.E.M.: Department of Chemistry, Indiana University, Bloomington, IN 47405. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant CHE-0552221.

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REFERENCES

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