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Vibrationally resolved photoelectron spectrum of ZnBr−
Journal of Molecular Spectroscopy 357 (2019) 38–40
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Vibrationally resolved photoelectron spectrum of ZnBr Jarrett L. Mason, Joshua C. Ewigleben, Caroline Chick Jarrold ⇑ Department of Chemistry, Indiana University, 800 East Kirkwood Ave., Bloomington, IN 47405, United States
a r t i c l e
i n f o
Article history: Received 7 January 2019 In revised form 13 February 2019 Accepted 13 February 2019 Available online 14 February 2019 Keywords: Zinc bromide radical Zinc. halides Anion photoelectron spectroscopy Vibrational spacings
a b s t r a c t Determination of the vibrational frequency of the ZnBr radical has been elusive, having previously been reported as 220 cm1 and 318 cm1 by different investigators. Recently, Burton and Ziurys reported a rotational spectrum of ZnBr from which they were able to estimate a harmonic frequency of 284 cm1 and an anharmonicity of 1.0 cm1. In this communication, we report a direct measurement of the vibrational spacing of ZnBr by anion photoelectron spectroscopy of ZnBr. We determine the neutral vibrational frequency to be 280 ± 5 cm1 (xexe = 1.5 ± 0.5 cm1) and the anion vibrational frequency to be 120 ± 10 cm1. In addition, we measure the electron affinity of the neutral radical to be 2.45 ± 0.06 eV, and determine that the bond dissociation energy of the anion is lower than that of the neutral by 0.91 ± 0.06 eV. Ó 2019 Published by Elsevier Inc.
The family of zinc halides has long been known for applications in organosynthetic chemistry. ZnBr2 can function as an important reagent in generating synthetically useful organozinc complexes [1], a catalyst for a variety of products including styrene carbonate [2] and diastereomerically enriched spirocyclic diols [3], and an electroinitiation agent in homopolymerization [4]. In addition, some cross-coupling reactions of alkyl electrophiles and alkyl zinc halides seem to follow a radical pathway [5]. Beyond this synthetic utility, ZnBr is of interest in the design and optimization of electrolyte batteries [6,7]. As such, it is of particular interest to definitively establish an in-depth characterization of the fundamental properties of this system. Surprisingly, the vibrational frequency of this diatomic has been somewhat of a slow-burning point of contention. A UV spectrum reported in 1971 gives ground 2R+ state and excited 2P1/2 and 2 P3/2 state harmonic frequencies of 318 cm1, 350 cm1, and 358 cm1, respectively, with anharmonicities for all approximated to be 2 cm1 [8]. A matrix isolation Raman spectroscopy study measured the energy between the v = 0 and v = 1 vibrational levels of ZnBr to be significantly lower, 220 cm1 [9]. Recently, Burton and Ziurys reported a rotational spectrum of several isotopomers of ZnBr, from which they determined an equilibrium bond distance of 2.268 Å, approximated the harmonic frequency as 284 cm1 and anharmonicity of 1.0 cm1, and a bond dissociation energy of 2.45 eV [10]. Their results supported the previously reported multireference configuration interaction (MRCI) calculations, which predicted a frequency of 267.2 cm1, with an equilibrium bond ⇑ Corresponding author. E-mail address: [email protected] (C.C. Jarrold). https://doi.org/10.1016/j.jms.2019.02.003 0022-2852/Ó 2019 Published by Elsevier Inc.
distance of 2.341 Å [11]. The more recent results give vibrational frequencies that lie between the measured values, and while they are reliable, they do not directly measure the vibrational level spacing of ZnBr. In this communication, we report the photoelectron (PE) spectrum of ZnBr, which exhibits an extended vibrational progression with a spacing of approximately 275 cm1. The progression is congested with significant contributions from vibrationally excited levels of the anion, which appears to have a much lower vibrational frequency. Indeed, we determine that the bond dissociation energy of the anion is approximately 0.9 to 1.0 eV lower than the neutral. The neutral bond dissociation energy was previously calculated to be 2.03 eV [11], and approximated by Burton and Ziurys to be 2.45 eV [10]. The anion photoelectron spectrometer used in this study has been described in detail previously [12]. ZnBr was generated using a laser ablation/pulsed molecular beam valve source [13], with a compressed Zn/ZnBr2 powder target of roughly 85% Zn and 15% ZnBr2 by mass (Alfa Aesar 99.9%, 98% respectively). Approximately 4–5 mJ/pulse of the second harmonic output (532.1 nm, 2.330 eV) of a Nd:YAG laser operated at 30 Hz was focused onto the surface of the target, and the resulting plasma was entrained in a pulse of ultra-high purity helium issued from a pulsed molecular beam valve. The gas mixture expanded into a vacuum chamber, and was collimated by a skimmer into a timeof-flight mass spectrometer. Prior to colliding with an ion detector, the mass-coincident 64Zn81Br and 66Zn79Br isotopomers, which are the most abundant with about twice as many 64Zn81Br per 66Zn79Br present, were selectively photodetached with the third harmonic output (354.7 nm, 3.495 eV) of a second Nd:YAG laser at the
J.L. Mason et al. / Journal of Molecular Spectroscopy 357 (2019) 38–40
intersection of the ion drift tube and a second field-free drift tube, at the end of which is an electron detector. The drift times of the small fraction of photoelectrons that collided with the second dual microchannel detector assembly at the end of the drift tube were recorded on a digitizing oscilloscope. Electron drift times were converted to electron kinetic energy (eKE) calibrated against the well-known spectra of O and OH. The eKE values are related to the anion and neutral states via (2): anion e KE ¼ hm EAad Eneutral internal þ Einternal
ð1Þ
where EAad is the adiabatic electron affinity of the neutral, Eneutral internal is the internal energy of the neutral (electronic, vibrational, rotational) and Eanion internal is the internal energy of the anion (electronic, vibrational and rotational). As will be shown below, only the ground electronic states of ZnBr and ZnBr are sampled in this experiment, and because the experimental resolution is insufficient to resolve individual rotational transitions, the eKE distribution shows peaks associated with transitions to different vibrational levels of the neutral from the vibrational levels of the anion that are populated in the anion beam. The data presented show electron counts plotted as a function of electron binding energy, eBE:
e BE ¼ hm e KE
ð2Þ
The e BE values reflect the energy difference between the final neutral state and the initial anion state, and are independent of the photon energy used. Laboratory to center-of-mass frame corrections were made to the eKE (and eBE) values. The spectrum was accumulated for 3.7 million laser shots. In addition, the spectrum was measured with laser polarizations parallel (h = 0°) and perpendicular (h = 90°) to the electron drift tube. However, the intensity of the spectrum collected with perpendicular polarization had near zero intensity, yielding an anisotropy parameter, b(E) of 2. While high level calculations have already been reported on the ground and low lying excited neutral states, we performed DFT calculations on the anion and neutral as a general check on the change in frequency and bondlength. The GAUSSIAN 16 quantum chemistry package [14] was used to calculate optimized structures of anionic and neutral ZnBr. Calculations were carried out using the Dunning style aug-cc-pVQZ basis set for both Zn and Br. The computational results were compared to spectroscopic parameters from empirical simulations of the PE spectrum generated using home-written simulation code described previously [15].
Fig. 1. PE spectrum of ZnBr measured using 3.495 eV photon energy.
39
Fig. 1 shows the PE spectrum of ZnBr measured with 3.495 eV photon energy. The origin is difficult to identify because of hot band congestion at the low electron binding energy side of the spectrum, the result of hot source conditions necessary for the production of sufficient quantities of ZnBr. If we were to overlay the vibrational manifold of the previously measured PE spectrum of ZnF with the vibrational manifold of ZnBr, the origin would be near 2.57 eV (a comparison of the vibrational manifolds of the two zinc halides is included in the supplementary material). However, based on simulations (vide infra), the actual origin is lower. Note that the electron affinity of ZnF was determined to be 2.087 eV [16]. The EA of ZnBr is clearly higher, despite the fact that the electron affinity of Br (3.3636 eV) is slightly lower than that of F (3.4012 eV) [17]. We performed a series of simulations with origins set to energies ranging from 2.395 to 2.563 eV, adjusting the bondlength change between the anion and neutral states to match the overall vibrational manifold, and vibrational frequency and anharmonicity to best fit the vibrational spacing. Selecting the 2.563 eV origin did not give an overall satisfactory fit because the intensity of the signal below 2.42 eV could not be matched, even at very high simulation temperatures (e.g., 1000 K). Better overall agreement was achieved with origins in the 2.40 to 2.47 eV range, and we conservatively report the electron affinity as 2.45 ± 0.06 eV. The simulation shown in Fig. 2 assumes an origin of 2.46 eV, a neutral harmonic frequency of 284 cm1, an anharmonicity of 1.5 cm1, a vibrational temperature of 400 K, an anion frequency of 125 cm1, and a bondlength change of 0.24 Å. Reasonable fits were also achieved with an origin set to 2.395 and a vibrational frequency of 280 cm1 (anharmonicity of 1 cm1). With higher origins, frequencies as low as 275 cm1 (anharmonicities from 1 to 1.5 cm1) provided a reasonable fit to the resolved structure. Note that the simulated progression shown in Fig. 2 is truncated at the high eBE side of the band due to the limited vibrational levels in the code (v00 = 0–6; v0 = 0–12). Table 1 summarizes the spectroscopic constants with conservative errors determined from the spectral fits. That is, the vibrational frequencies were varied around the central value reported to the point where the fit was no longer reasonable to determine the range of acceptable values reported. While the vibrational frequency of the anion used in the simulations seems particularly low, it is useful to consider that within the ionic bonding picture, the doubly occupied HOMO of the anion
Fig. 2. Empirical simulation of the PE spectrum assuming an origin of 2.46 eV, x0e = of 284 cm1, xex0e = 1.5 cm1, Tvib = 400 K, x00e (anion) = 125 cm1, DrZnBr = 0.24 Å.
40
J.L. Mason et al. / Journal of Molecular Spectroscopy 357 (2019) 38–40
Table 1 DFT-calculated and experimental spectroscopic constants. Calc’d Rel. ZPE corrected E (eV) ZnBr R ZnBr1R+ 2
a b
+
2.31 0.0
Exp. EA 2.45 ± 0.06 –
Exp. Zn-Br bondlengtha (Å)
Calc’d rZnBr (Å) 2.325 2.750
b
2.268 2.51 ± 0.06
Calc’d xe (cm1) b
260 111
Exp xe (cm1)
Exp. xexe (cm1)
280 ± 5 120 ± 10
1.5 ± 0.5 –
From Ref. [10]. Ref. [11] gives rZnBr = 2.341 Å, xe = 267.2 cm1.
is a pz polarized Zn 4s orbital. This orbital is the antibonding partner to the Zn 4s- Br 4pz bonding orbital, with the Br 4pz contributing much more heavily to the bonding orbital in the ionic picture. Note that the Zn 3d [10] subshell is core-like and does not participate in bonding. Also summarized in Table 1 are results of DFT calculations, which predict 260 cm1 and 111 cm1 vibrational frequencies for the neutral and anion respectively, and a difference in bond distance of 0.425 Å. The calculated neutral bond distance and vibrational frequency are close to the MRCI calculated values, and the calculated electron affinity, 2.31 eV, is slightly below the range of spectral origins used for generation of satisfactory simulations (2.395–2.465 eV). Because the computational method systematically over-stabilizes higher spin states [18], we expect the calculated electron affinity to be lower than the actual value. However, the calculated ZnBr bond distance is significantly longer than what is suggested from the simulations. Additional insight into bonding in the ZnBr anion can be drawn from the relationship derived from ZnBr ? Zn + Br + e,
DZnBr þ EABr ¼ EAZnBr þ DZnBr 0 0
ð3Þ
The electron affinity of Br is 3.363 eV [17], so assuming a ZnBr electron affinity of 2.45 ± 0.06 eV, DZnBr 0
is 0.91 ± 0.06 eV lower
DZnBr , 0
than which was calculated to be 2.03 eV [11], and estimated to be 2.45 eV [10] based on the Morse oscillator relationship between the equilibrium rotational constant, centrifugal correction term and the harmonic frequency and anharmonicity, which give
the Morse oscillator dissociation energy [19]. Therefore, DZnBr 0 can be reasonably estimated as 1.5 eV, and possibly as low as 1.1 eV. A diagram showing Morse potentials for the anion and neutral, illustrating the relationship in Eq. (3) is included in the supplementary material. As noted above, the ZnF PE spectrum was reported previously [16]. The neutral and anion frequencies of ZnF were determined to be 620 cm1 and 420 cm1, respectively. If we simply scaled the frequency by the square root of the reduced mass (lZnF = 14.7 amu; lZnBr = 35.9 amu) the ZnBr neutral and anion frequencies would be 400 cm1 and 268 cm1, respectively. In addition, in terms of mass weighted vibrational coordinate displacement, the ZnF spectrum was consistent with 0.57 Åamu1/2 displacement between the anion and neutral, while for ZnBr, the displacement is 1.4 Åamu1/2 (vide supra). All of these observations point to a very shallow internuclear potential for the anion. As noted by Burton and Ziurys, the ZnBr bond has more covalent character than the ZnF bond [10], and our results support the doubly occupied r⁄ molecular orbital in ZnBr has more antibonding character than the analogous ZnF r⁄ orbital. While the harmonic frequency of ZnBr has been a point of uncertainty for several decades now, this communication reports a direct measurement of the vibrational levels of the ZnBr radical collected by PE spectroscopy of the corresponding anion. ZnBr exhibits an extended vibrational progression of 280 ± 5 cm1 and anharmonicity of 1.5 ± 0.5 cm1. Furthermore, we have determined conservative values for both the electron affinity of ZnBr, 2.45(6) eV, and the difference in bond dissociation energy of the anion and neutral, with the anion bond dissociation energy being 0.91 ± 0.06 eV lower than the neutral. The lower bond dissociation energy is reflected in the anion vibrational frequency,
120 ± 10 cm1, and a bondlength elongated by over 0.2 Å. The values reported here are wholly consistent with and complementary to those calculated from the rotational constants in the recently reported pure rotational spectrum of neutral ZnBr [10]. Burton and Ziurys estimated a harmonic frequency of 284 cm1 and anharmonicity of 1.0 cm1, from which they calculated a bond dissociation energy of 2.45 eV, giving an anion bond dissociation of approximately 1.5 eV. Acknowledgments This work was supported by the Indiana University College of Arts and Sciences, Indiana University, United States. CCJ and JLM appreciate useful conversations with Mr. Mark Burton and, and thank Prof. Lucy Ziurys for sharing unpublished results. Appendix A. Supplementary material Supplementary material includes a comparison of the spectra of ZnF and ZnBr as well as an energy level diagram showing Morse potentials for anionic and neutral ZnBr. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jms.2019.02.003. References [1] Y. Zhou, W.X. Zhang, Z. Xi, Organometallics 31 (2012) 5546. [2] J. Sun, S.I. Fujita, F. Zhao, M. Arai, Green Chem. 6 (2004) 613. [3] Y.Q. Tu, C.A. Fan, S.K. Ren, A.S.C. Chan, J. Chem. Soc., Perkin Trans. I. 22 (2000) 3791. [4] D.C. Phillips, D.H. Davies, J.B.D. Smith, Macromolecules. 5 (1972) 674. [5] V.B. Phapale, E. Buñuel, G.M. Iglesias, D.J. Cárdenas, Angew. Chem. 119 (2007) 8946. [6] M.C. Wu, T.S. Zhao, H.R. Jiang, Y.K. Zeng, Y.X. Ren, J. Power Sources 355 (2017) 62. [7] S. Suresh, T. Kesavan, Y. Munaiah, I. Arulraj, S. Dheenadayala, P. Ragupathy, RSC Adv. 4 (2014) 37947. [8] R.K. Gosavi, G. Greig, P.J. Young, O.P. Strausz, J. Chem. Phys. 54 (1971) 983. [9] A. Givan, A. Loewenschuss, J. Mol. Struct. 78 (1982) 299. [10] M.A. Burton, L.M. Ziurys, J. Chem. Phys. 150 (2019) 034303. [11] S. Elmoussaoui, M. Korec, Comp. Theor. Chem. 1068 (2015) 42. [12] V.D. Moravec, C.C. Jarrold, J. Chem. Phys. 108 (1998) 1804. [13] S.E. Waller, J.E. Mann, C.C. Jarrold, J. Phys. Chem. A 117 (2013) 1765. [14] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian, Inc., Gaussian 16 RevA.03 Wallingford CT, 2016. [15] R.N. Schaugaard, J.E. Topolski, M. Ray, K. Raghavachari, C.C. Jarrold, J. Chem. Phys. 148 (2018) 054308. [16] V.D. Moravec, S.A. Klopcic, B. Chatterjee, C.C. Jarrold, Chem. Phys. Lett. 341 (2001) 313. [17] ‘‘Electron Affinities,” in: John R. Rumble (ed.), CRC Handbook of Chemistry and Physics, 99th Edition (Internet Version 2018), CRC Press/Taylor & Francis, Boca Raton, FL. [18] J. Harvey, DFT computation of relative spin-state energetics of transition metal compounds, Structure and Bonding, Springer, Berlin, 2004. [19] W. Gordy, R.L. Cook, Microwave Molecular Spectra, Wiley, New York, 1984.
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Vibrationally resolved photoelectron spectrum of ZnBr Jarrett L. Mason, Joshua C. Ewigleben, Caroline Chick Jarrold ⇑ Department of Chemistry, Indiana University, 800 East Kirkwood Ave., Bloomington, IN 47405, United States
a r t i c l e
i n f o
Article history: Received 7 January 2019 In revised form 13 February 2019 Accepted 13 February 2019 Available online 14 February 2019 Keywords: Zinc bromide radical Zinc. halides Anion photoelectron spectroscopy Vibrational spacings
a b s t r a c t Determination of the vibrational frequency of the ZnBr radical has been elusive, having previously been reported as 220 cm1 and 318 cm1 by different investigators. Recently, Burton and Ziurys reported a rotational spectrum of ZnBr from which they were able to estimate a harmonic frequency of 284 cm1 and an anharmonicity of 1.0 cm1. In this communication, we report a direct measurement of the vibrational spacing of ZnBr by anion photoelectron spectroscopy of ZnBr. We determine the neutral vibrational frequency to be 280 ± 5 cm1 (xexe = 1.5 ± 0.5 cm1) and the anion vibrational frequency to be 120 ± 10 cm1. In addition, we measure the electron affinity of the neutral radical to be 2.45 ± 0.06 eV, and determine that the bond dissociation energy of the anion is lower than that of the neutral by 0.91 ± 0.06 eV. Ó 2019 Published by Elsevier Inc.
The family of zinc halides has long been known for applications in organosynthetic chemistry. ZnBr2 can function as an important reagent in generating synthetically useful organozinc complexes [1], a catalyst for a variety of products including styrene carbonate [2] and diastereomerically enriched spirocyclic diols [3], and an electroinitiation agent in homopolymerization [4]. In addition, some cross-coupling reactions of alkyl electrophiles and alkyl zinc halides seem to follow a radical pathway [5]. Beyond this synthetic utility, ZnBr is of interest in the design and optimization of electrolyte batteries [6,7]. As such, it is of particular interest to definitively establish an in-depth characterization of the fundamental properties of this system. Surprisingly, the vibrational frequency of this diatomic has been somewhat of a slow-burning point of contention. A UV spectrum reported in 1971 gives ground 2R+ state and excited 2P1/2 and 2 P3/2 state harmonic frequencies of 318 cm1, 350 cm1, and 358 cm1, respectively, with anharmonicities for all approximated to be 2 cm1 [8]. A matrix isolation Raman spectroscopy study measured the energy between the v = 0 and v = 1 vibrational levels of ZnBr to be significantly lower, 220 cm1 [9]. Recently, Burton and Ziurys reported a rotational spectrum of several isotopomers of ZnBr, from which they determined an equilibrium bond distance of 2.268 Å, approximated the harmonic frequency as 284 cm1 and anharmonicity of 1.0 cm1, and a bond dissociation energy of 2.45 eV [10]. Their results supported the previously reported multireference configuration interaction (MRCI) calculations, which predicted a frequency of 267.2 cm1, with an equilibrium bond ⇑ Corresponding author. E-mail address: [email protected] (C.C. Jarrold). https://doi.org/10.1016/j.jms.2019.02.003 0022-2852/Ó 2019 Published by Elsevier Inc.
distance of 2.341 Å [11]. The more recent results give vibrational frequencies that lie between the measured values, and while they are reliable, they do not directly measure the vibrational level spacing of ZnBr. In this communication, we report the photoelectron (PE) spectrum of ZnBr, which exhibits an extended vibrational progression with a spacing of approximately 275 cm1. The progression is congested with significant contributions from vibrationally excited levels of the anion, which appears to have a much lower vibrational frequency. Indeed, we determine that the bond dissociation energy of the anion is approximately 0.9 to 1.0 eV lower than the neutral. The neutral bond dissociation energy was previously calculated to be 2.03 eV [11], and approximated by Burton and Ziurys to be 2.45 eV [10]. The anion photoelectron spectrometer used in this study has been described in detail previously [12]. ZnBr was generated using a laser ablation/pulsed molecular beam valve source [13], with a compressed Zn/ZnBr2 powder target of roughly 85% Zn and 15% ZnBr2 by mass (Alfa Aesar 99.9%, 98% respectively). Approximately 4–5 mJ/pulse of the second harmonic output (532.1 nm, 2.330 eV) of a Nd:YAG laser operated at 30 Hz was focused onto the surface of the target, and the resulting plasma was entrained in a pulse of ultra-high purity helium issued from a pulsed molecular beam valve. The gas mixture expanded into a vacuum chamber, and was collimated by a skimmer into a timeof-flight mass spectrometer. Prior to colliding with an ion detector, the mass-coincident 64Zn81Br and 66Zn79Br isotopomers, which are the most abundant with about twice as many 64Zn81Br per 66Zn79Br present, were selectively photodetached with the third harmonic output (354.7 nm, 3.495 eV) of a second Nd:YAG laser at the
J.L. Mason et al. / Journal of Molecular Spectroscopy 357 (2019) 38–40
intersection of the ion drift tube and a second field-free drift tube, at the end of which is an electron detector. The drift times of the small fraction of photoelectrons that collided with the second dual microchannel detector assembly at the end of the drift tube were recorded on a digitizing oscilloscope. Electron drift times were converted to electron kinetic energy (eKE) calibrated against the well-known spectra of O and OH. The eKE values are related to the anion and neutral states via (2): anion e KE ¼ hm EAad Eneutral internal þ Einternal
ð1Þ
where EAad is the adiabatic electron affinity of the neutral, Eneutral internal is the internal energy of the neutral (electronic, vibrational, rotational) and Eanion internal is the internal energy of the anion (electronic, vibrational and rotational). As will be shown below, only the ground electronic states of ZnBr and ZnBr are sampled in this experiment, and because the experimental resolution is insufficient to resolve individual rotational transitions, the eKE distribution shows peaks associated with transitions to different vibrational levels of the neutral from the vibrational levels of the anion that are populated in the anion beam. The data presented show electron counts plotted as a function of electron binding energy, eBE:
e BE ¼ hm e KE
ð2Þ
The e BE values reflect the energy difference between the final neutral state and the initial anion state, and are independent of the photon energy used. Laboratory to center-of-mass frame corrections were made to the eKE (and eBE) values. The spectrum was accumulated for 3.7 million laser shots. In addition, the spectrum was measured with laser polarizations parallel (h = 0°) and perpendicular (h = 90°) to the electron drift tube. However, the intensity of the spectrum collected with perpendicular polarization had near zero intensity, yielding an anisotropy parameter, b(E) of 2. While high level calculations have already been reported on the ground and low lying excited neutral states, we performed DFT calculations on the anion and neutral as a general check on the change in frequency and bondlength. The GAUSSIAN 16 quantum chemistry package [14] was used to calculate optimized structures of anionic and neutral ZnBr. Calculations were carried out using the Dunning style aug-cc-pVQZ basis set for both Zn and Br. The computational results were compared to spectroscopic parameters from empirical simulations of the PE spectrum generated using home-written simulation code described previously [15].
Fig. 1. PE spectrum of ZnBr measured using 3.495 eV photon energy.
39
Fig. 1 shows the PE spectrum of ZnBr measured with 3.495 eV photon energy. The origin is difficult to identify because of hot band congestion at the low electron binding energy side of the spectrum, the result of hot source conditions necessary for the production of sufficient quantities of ZnBr. If we were to overlay the vibrational manifold of the previously measured PE spectrum of ZnF with the vibrational manifold of ZnBr, the origin would be near 2.57 eV (a comparison of the vibrational manifolds of the two zinc halides is included in the supplementary material). However, based on simulations (vide infra), the actual origin is lower. Note that the electron affinity of ZnF was determined to be 2.087 eV [16]. The EA of ZnBr is clearly higher, despite the fact that the electron affinity of Br (3.3636 eV) is slightly lower than that of F (3.4012 eV) [17]. We performed a series of simulations with origins set to energies ranging from 2.395 to 2.563 eV, adjusting the bondlength change between the anion and neutral states to match the overall vibrational manifold, and vibrational frequency and anharmonicity to best fit the vibrational spacing. Selecting the 2.563 eV origin did not give an overall satisfactory fit because the intensity of the signal below 2.42 eV could not be matched, even at very high simulation temperatures (e.g., 1000 K). Better overall agreement was achieved with origins in the 2.40 to 2.47 eV range, and we conservatively report the electron affinity as 2.45 ± 0.06 eV. The simulation shown in Fig. 2 assumes an origin of 2.46 eV, a neutral harmonic frequency of 284 cm1, an anharmonicity of 1.5 cm1, a vibrational temperature of 400 K, an anion frequency of 125 cm1, and a bondlength change of 0.24 Å. Reasonable fits were also achieved with an origin set to 2.395 and a vibrational frequency of 280 cm1 (anharmonicity of 1 cm1). With higher origins, frequencies as low as 275 cm1 (anharmonicities from 1 to 1.5 cm1) provided a reasonable fit to the resolved structure. Note that the simulated progression shown in Fig. 2 is truncated at the high eBE side of the band due to the limited vibrational levels in the code (v00 = 0–6; v0 = 0–12). Table 1 summarizes the spectroscopic constants with conservative errors determined from the spectral fits. That is, the vibrational frequencies were varied around the central value reported to the point where the fit was no longer reasonable to determine the range of acceptable values reported. While the vibrational frequency of the anion used in the simulations seems particularly low, it is useful to consider that within the ionic bonding picture, the doubly occupied HOMO of the anion
Fig. 2. Empirical simulation of the PE spectrum assuming an origin of 2.46 eV, x0e = of 284 cm1, xex0e = 1.5 cm1, Tvib = 400 K, x00e (anion) = 125 cm1, DrZnBr = 0.24 Å.
40
J.L. Mason et al. / Journal of Molecular Spectroscopy 357 (2019) 38–40
Table 1 DFT-calculated and experimental spectroscopic constants. Calc’d Rel. ZPE corrected E (eV) ZnBr R ZnBr1R+ 2
a b
+
2.31 0.0
Exp. EA 2.45 ± 0.06 –
Exp. Zn-Br bondlengtha (Å)
Calc’d rZnBr (Å) 2.325 2.750
b
2.268 2.51 ± 0.06
Calc’d xe (cm1) b
260 111
Exp xe (cm1)
Exp. xexe (cm1)
280 ± 5 120 ± 10
1.5 ± 0.5 –
From Ref. [10]. Ref. [11] gives rZnBr = 2.341 Å, xe = 267.2 cm1.
is a pz polarized Zn 4s orbital. This orbital is the antibonding partner to the Zn 4s- Br 4pz bonding orbital, with the Br 4pz contributing much more heavily to the bonding orbital in the ionic picture. Note that the Zn 3d [10] subshell is core-like and does not participate in bonding. Also summarized in Table 1 are results of DFT calculations, which predict 260 cm1 and 111 cm1 vibrational frequencies for the neutral and anion respectively, and a difference in bond distance of 0.425 Å. The calculated neutral bond distance and vibrational frequency are close to the MRCI calculated values, and the calculated electron affinity, 2.31 eV, is slightly below the range of spectral origins used for generation of satisfactory simulations (2.395–2.465 eV). Because the computational method systematically over-stabilizes higher spin states [18], we expect the calculated electron affinity to be lower than the actual value. However, the calculated ZnBr bond distance is significantly longer than what is suggested from the simulations. Additional insight into bonding in the ZnBr anion can be drawn from the relationship derived from ZnBr ? Zn + Br + e,
DZnBr þ EABr ¼ EAZnBr þ DZnBr 0 0
ð3Þ
The electron affinity of Br is 3.363 eV [17], so assuming a ZnBr electron affinity of 2.45 ± 0.06 eV, DZnBr 0
is 0.91 ± 0.06 eV lower
DZnBr , 0
than which was calculated to be 2.03 eV [11], and estimated to be 2.45 eV [10] based on the Morse oscillator relationship between the equilibrium rotational constant, centrifugal correction term and the harmonic frequency and anharmonicity, which give
the Morse oscillator dissociation energy [19]. Therefore, DZnBr 0 can be reasonably estimated as 1.5 eV, and possibly as low as 1.1 eV. A diagram showing Morse potentials for the anion and neutral, illustrating the relationship in Eq. (3) is included in the supplementary material. As noted above, the ZnF PE spectrum was reported previously [16]. The neutral and anion frequencies of ZnF were determined to be 620 cm1 and 420 cm1, respectively. If we simply scaled the frequency by the square root of the reduced mass (lZnF = 14.7 amu; lZnBr = 35.9 amu) the ZnBr neutral and anion frequencies would be 400 cm1 and 268 cm1, respectively. In addition, in terms of mass weighted vibrational coordinate displacement, the ZnF spectrum was consistent with 0.57 Åamu1/2 displacement between the anion and neutral, while for ZnBr, the displacement is 1.4 Åamu1/2 (vide supra). All of these observations point to a very shallow internuclear potential for the anion. As noted by Burton and Ziurys, the ZnBr bond has more covalent character than the ZnF bond [10], and our results support the doubly occupied r⁄ molecular orbital in ZnBr has more antibonding character than the analogous ZnF r⁄ orbital. While the harmonic frequency of ZnBr has been a point of uncertainty for several decades now, this communication reports a direct measurement of the vibrational levels of the ZnBr radical collected by PE spectroscopy of the corresponding anion. ZnBr exhibits an extended vibrational progression of 280 ± 5 cm1 and anharmonicity of 1.5 ± 0.5 cm1. Furthermore, we have determined conservative values for both the electron affinity of ZnBr, 2.45(6) eV, and the difference in bond dissociation energy of the anion and neutral, with the anion bond dissociation energy being 0.91 ± 0.06 eV lower than the neutral. The lower bond dissociation energy is reflected in the anion vibrational frequency,
120 ± 10 cm1, and a bondlength elongated by over 0.2 Å. The values reported here are wholly consistent with and complementary to those calculated from the rotational constants in the recently reported pure rotational spectrum of neutral ZnBr [10]. Burton and Ziurys estimated a harmonic frequency of 284 cm1 and anharmonicity of 1.0 cm1, from which they calculated a bond dissociation energy of 2.45 eV, giving an anion bond dissociation of approximately 1.5 eV. Acknowledgments This work was supported by the Indiana University College of Arts and Sciences, Indiana University, United States. CCJ and JLM appreciate useful conversations with Mr. Mark Burton and, and thank Prof. Lucy Ziurys for sharing unpublished results. Appendix A. Supplementary material Supplementary material includes a comparison of the spectra of ZnF and ZnBr as well as an energy level diagram showing Morse potentials for anionic and neutral ZnBr. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jms.2019.02.003. References [1] Y. Zhou, W.X. Zhang, Z. Xi, Organometallics 31 (2012) 5546. [2] J. Sun, S.I. Fujita, F. Zhao, M. Arai, Green Chem. 6 (2004) 613. [3] Y.Q. Tu, C.A. Fan, S.K. Ren, A.S.C. Chan, J. Chem. Soc., Perkin Trans. I. 22 (2000) 3791. [4] D.C. Phillips, D.H. Davies, J.B.D. Smith, Macromolecules. 5 (1972) 674. [5] V.B. Phapale, E. Buñuel, G.M. Iglesias, D.J. Cárdenas, Angew. Chem. 119 (2007) 8946. [6] M.C. Wu, T.S. Zhao, H.R. Jiang, Y.K. Zeng, Y.X. Ren, J. Power Sources 355 (2017) 62. [7] S. Suresh, T. Kesavan, Y. Munaiah, I. Arulraj, S. Dheenadayala, P. Ragupathy, RSC Adv. 4 (2014) 37947. [8] R.K. Gosavi, G. Greig, P.J. Young, O.P. Strausz, J. Chem. Phys. 54 (1971) 983. [9] A. Givan, A. Loewenschuss, J. Mol. Struct. 78 (1982) 299. [10] M.A. Burton, L.M. Ziurys, J. Chem. Phys. 150 (2019) 034303. [11] S. Elmoussaoui, M. Korec, Comp. Theor. Chem. 1068 (2015) 42. [12] V.D. Moravec, C.C. Jarrold, J. Chem. Phys. 108 (1998) 1804. [13] S.E. Waller, J.E. Mann, C.C. Jarrold, J. Phys. Chem. A 117 (2013) 1765. [14] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian, Inc., Gaussian 16 RevA.03 Wallingford CT, 2016. [15] R.N. Schaugaard, J.E. Topolski, M. Ray, K. Raghavachari, C.C. Jarrold, J. Chem. Phys. 148 (2018) 054308. [16] V.D. Moravec, S.A. Klopcic, B. Chatterjee, C.C. Jarrold, Chem. Phys. Lett. 341 (2001) 313. [17] ‘‘Electron Affinities,” in: John R. Rumble (ed.), CRC Handbook of Chemistry and Physics, 99th Edition (Internet Version 2018), CRC Press/Taylor & Francis, Boca Raton, FL. [18] J. Harvey, DFT computation of relative spin-state energetics of transition metal compounds, Structure and Bonding, Springer, Berlin, 2004. [19] W. Gordy, R.L. Cook, Microwave Molecular Spectra, Wiley, New York, 1984.