Photodissociation spectroscopy and dynamics of Mg+-formaldehyde

Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI. J Chem Phys 2023;158:054303. [PMID: 36754811 DOI: 10.1063/5.0134668] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The bond strength and photodissociation dynamics of MgI⁺ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm^-1, the photodissociation spectrum of MgI⁺ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg⁺ (²S) + I (²P_3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D₀(Mg⁺-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm^-1). At photon energies of 33 000-41 000 cm^-1, exclusively I⁺ fragments are formed. Over most of this region, the formation of I⁺ is not energetically allowed via one-photon absorption from the ground state of MgI⁺. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm^-1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm^-1. This is consistent with absorption to the (3) Ω = 0⁺ state from ν = 0, 1 of the (1) Ω = 0⁺ ground state; from the (3) Ω = 0⁺ state, absorption of a second photon results in dissociation to Mg* (³P^° _J) + I⁺ (³P_J). From 38 500 to 41 000 cm^-1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI⁺ at low energy and ground state MgI⁺ at higher energy to form Mg (¹S) + I⁺ (³P_J) products.

UNUSUAL LASER-INDUCED ABSORPTIONS OF Ca+-FORMALDEHYDE: MOLECULAR ORBITAL INTERACTION

We have theoretically studied the photodissociation spectroscopy of Ca +-formaldehyde complex using the TD-B2-PLYP method. The SDD pseudopotential and basis sets for Ca and 6-31++G (2df, 2pd) basis sets for C , H , and O atoms were employed in all calculations. In this way, we have reassigned the photodissociation spectroscopy of this complex. All experimentally observed spectral features can be well explained by our calculation. Besides the charge–dipole interaction, a strong molecule–orbital interaction also exists in the excited states and plays an important role in photoexcitation of the Ca+–CH2O complex.

Reaction rate constants and mechanistic detail of the Ni+ + butanone reaction

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm⁻¹). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH₃ or C-C₂H₅ σ-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each σ-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm⁻¹. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C₂H₅) = (0.92 ± 0.08) × 10⁵ s⁻¹ and k(act)(C-CH₃) = (0.37 ± 0.03) × 10⁵ s⁻¹. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C₂H₅ σ-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products.

Pyridyne radical cations produced by photodissociation of Mg˙+(multifluoro-pyridine) complexes: A combined experimental and theoretical study

Gas phase complexes Mg*+ (2,6-difluoropyridine) (1) and Mg*+ (pentafluoropyridine) (2) have been subjected to photodissociation in the spectral range of approximately 230-440 nm. Except for the evaporative photofragment Mg*+ , the primary photoproduct for is C(5)H(3)N*(+), which is associated with the rupture of two C-F bonds by the photoexcited Mg*+ , forming very stable MgF(2). In contrast, the direct loss of MgF(+) is more favorable for due to fluorine substitution. Given enough energy, C(5)H(3)N*(+) can undergo decomposition to form C(4)H(2)*(+) and HCN. These results are very different from those for Mg*+ (2-fluoropyridine), highlighting the significance of the additional F at C6 of and . Density functional theory (DFT) calculations have been employed to examine the geometries and energetics of the complexes as well as relevant reaction mechanisms. All of the complexes feature the direct attachment of Mg*+ to the N atom. The key intermediate is found to be FMg(+) (C(5)H(x)F(4-x)N) (x = 3 or 0), which can lead to the formation of MgF(+) directly or MgF(2) through activation of another C-F bond adjacent to N, producing the pyridyne radical cations. However, hydrogen-transfer prior to the rupture of the second C-F bond followed by ring-opening of C(5)H(3)N*(+) may result in the formation of chain forms of C(5)H(3)N*(+). The influence of the fluorine substitution on the competition of the two routes have been demonstrated.

Photodissociation spectroscopy of the Mg+-acetic acid complex

We have studied the structure and photodissociation of Mg(+)-acetic acid clusters. Ab initio calculations suggest four relatively strongly bound ground state isomers for the [MgC(2)H(4)O(2)](+) complex. These isomers include the cis and trans forms of the Mg(+)-acetic acid association complex with Mg(+) bonded to the carbonyl O atom of acetic acid, the Mg(+)-acetic acid association complex with Mg(+) bonded to the hydroxyl O atom of acetic acid, or to a Mg(+)-ethenediol association complex. Photodissociation through the Mg(+)-based 3p<--3s absorption bands in the near UV leads to direct (nonreactive) and reactive dissociation products: Mg(+), MgOH(+), Mg(H(2)O)(+), CH(3)CO(+), and MgCH(3) (+). At low energies the dominant reactive quenching pathway is through dehydration to Mg(H(2)O)(+), but additional reaction channels involving C-H and C-C bond activation are also open at higher energies.

Photodissociation spectroscopy of Zn+(H2O) and Zn+(D2O)

We report on a study of the photodissociation spectroscopy of weakly bound Zn+(H2O) and Zn+(D2O) complexes. The work is supported by ab initio electronic structure calculations of the ground and low-lying excited energy surfaces. We assign two molecular absorption bands in the near UV correlating to Zn+ (4s-4p)-based transitions, and identify vibrational progressions associated with both intermolecular and intramolecular vibrational modes of the cluster. Partially resolved rotational structure is consistent with a C(2V) equilibrium complex geometry. Experimental spectroscopic constants are in very good agreement with ab initio theoretical predictions. Results are compared with previous work on main group and transition metal ion-H2O clusters.

Comparative studies of the photoinduced reactions in the Mg+–SCNC2H5 and Mg+–NCSC2H5 complexes

The photoinduced reactions of the complexes Mg+-SCNC2H5 and Mg+-NCSC2H5 are studied comparatively in the spectral range of 230-440 nm. One-photon excitation of the complexes through the Mg+ chromophore (3 2P <-- 3 2S) gives rise to the evaporative fragment as well as the molecular activation and charge transfer products. The action spectra of the complexes consist of three broad peaks for Mg+-SCNC2H5 and two for Mg+-NCSC2H5, which accord with the structures obtained from quantum mechanics calculations. These calculations reveal two association isomers for Mg+-SCNC2H5: one is with Mg+ being linked to the S atom and the other to the N atom. The former is more stable than the latter by only 0.23 eV. Both of the isomers have been shown to exist in the complex source employed in our experiments. On the other hand, only one stable structure is found for the complex Mg+-NCSC2H5 characterized by the Mg+-N linkage. In general, the photofragments are dominated by Mg+ at lambda > 400 nm, which decreases with decreasing wavelength accompanied by the increase in other photoproducts. In addition, the branching ratios of Mg+ to other photoproducts are nearly constant in the short wavelength region but decrease with decreasing wavelength. The observed photoreactions have been reasonably explained.

Unusual Chemistry of the Complex Mg•+(2-Fluoropyridine) Activated by the Photoexcitation of Mg•+

The photochemistry of a gas-phase complex, Mg*(+)(2-fluoropyridine), has been studied in the spectral range of approximately 230-440 nm with a molecular beam coupled with a time-of-flight mass spectrometer. Surprisingly rich chemistry has been observed. Aside from the evaporative photofragment, Mg*(+), an abundant photoproduct, C(4)H(4)*(+), is observed after the electronic excitation of Mg(+). The formation of this photoproduct is associated with the loss of a stable species, CN[bond]Mg[bond]F. Also identified in this work are reactive pathways that occur with the elimination of HCN, HF, or MgF from the complex. The observed photoreactions have been examined in detail using quantum mechanics methods. A distinct structural feature of the complex is the direct attachment of Mg*(+) to the N atom of fluoropyridine due to the strong electrostatic interaction. The key to the rich photochemistry is the formation of the FMg(+)(C(5)H(4)N) intermediate, through facile fluorine migration. Plausible photoreaction mechanisms have been proposed. These mechanisms account for the evolution of the energized complex with the pre-defined structure en route to the target photoproducts that we have detected.