Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase

Beyer and coworkers have similarly studied a range of hydrated transition metal-CO 2 clusters by employing IRPD within an ion cyclotron resonance mass spectrometer. 22,23 ecently, timeresolved 2D-IR spectroscopy has been applied to watch energy flow and frustrated intramolecular vibrational redistribution (IVR) in Re(CO) n (CH 3 CN) m + complexes. 24 have previously reported detailed IRPD studies of mixed CO and N 2 O complexes of Au + and Rh + , characterizing structures and comparing fragmentation patterns following excitation of the near degenerate C≡O and N 2 O N=N stretching modes, respectively. 25,26 ualitatively different behaviour is observed in each with the Au + complexes exhibiting purely nonclassical carbonyl bonding whilst cooperative effects in Rh + result in a mix of classical and non classical binding. 27,28 Th most significant differences, however were observed in the fragmentation dynamics with Au(CO) n (N 2 O) m + exhibiting unexpected non-statistical fragmentation with preferential CO loss contrary to expectations based on relative ligand binding energies.
Both Au(CO) x + and Au(N 2 O) y + complexes have been studied in detail by IRPD spectroscopy. 28,29 mixed-ligand complexes, the stronger binding of the CO leads to Au(CO) x + core structures with resulting blue-shifted C≡O stretches (2230 ± 10 cm -1 ).N 2 O binds more weakly to this core with the N=N stretch observed close to that in free N 2 O (2223 cm -1 ). 25 Our inability to resolve the CO and N=N stretches presented an ambiguity in our previous spectral assignment which we address here.In this work, we report the IRPD spectroscopy of isotopically-selected Au( 12/13 CO) n (N 2 O) m + complexes.The fragmentation of these species provides insight into the dynamics of these processes and restricted energy flow within these complexes.

II. Experimental and Computational Methods
The spectrometer used in this work has been described in detail previously. 26,30,31 Al gas mixes used were 1% CO: 5% N 2 O in Ar.In order to generate complexes with different 13 CO / 12 CO ligand fractions the CO used ranged from 100% 12 CO to 100% 13 CO.The mass spectrum of species generated in a 12 CO:N 2 O:Ar mix has been published previously 25 and so Figure 1 shows a representative time-of-flight mass spectrum of Au( 13 CO) n (N 2 O) m + produced by laser ablation of an Au target in presence of 1% 13 CO, 5% N 2 O gas mix in Ar and introduced at a backing pressure of 6 bar (see Figure S1 in the supporting information for more a detailed spectrum).The distribution of species produced is qualitatively similar to that in our earlier work on the 12 CO mixed-ligand system. 25e strongest ion signal observed is the Au( 13 CO) 2 + species whose stability is an example of the Orgel effect. 32,33 ther abundant species observed include Au( 13 CO) n (N 2 O) m + (n = 0-5, m = 0-3).
Mass-selected infrared action spectra were recorded by selecting parent ions using a quadrupole mass filter.In some cases involving mixed 12 S4).Gating the signal in different mass channels allows fragment-specific spectra to be recorded yielding information on the energy flow within the complex.The resulting fragmentation dynamics have been the focus of previous studies and we return to them here with isotopologue selectivity.First, however, it is necessary to understand the spectroscopy of the isotopically-labelled complexes.To aid structural assignment, the IRPD spectra recorded are compared with those simulated for the lowest-energy isomeric structures determined at the B3P86-Def2TZVP level of theory 28,34,35 which proved effective in our previous studies.All electronic structure calculations presented here were completed using Gaussian 16 program. 36Scalar relativistic effects were included with the use of the ECP60 effective core potential for the Au ion.All low-lying structures identified were singlet states.Calculated line spectra have been convoluted with Lorentzian line-shapes to help comparison with the experimental data.Calculated harmonic frequencies rarely match observed bands well and it is conventional to scale calculated vibrational frequencies 37 for comparison with experiment.This is usually done by scaling the calculated frequency of a fundamental band to its known experimental value.This is challenging in systems with multiple (different) ligands.To a good approximation, as expected in relatively weakly-bound complexes, the bands observed in the complexes with those simulated for the lowest energy calculated structures.This spectral region covers i) the anti-phase and in-phase CO stretching bands in the Au(CO) 2 + core (labelled A, and B, respectively) and ii) the N 2 O N=N stretch fundamental (labelled C).For brevity, we introduce a new nomenclature in which structures are labelled based on the number of 13 CO or 12 CO equivalents, so the Au( 13 CO)( 12 CO)(N 2 O) + isotopologue is labelled 13 C 12 CN, and the Au( 13 CO) 2 (N 2 O) + complex 13 C2N.The spectrum of the 12 C2N complex is reproduced from our original article. 25In all three isotopologues, only the simple N 2 O loss channel is observed, reflecting the highly stable Au(CO) 2 + core to which the N 2 O binds more weakly.Hence all the spectra shown are recorded as enhancements, from a zero baseline, in the Au(CO) 2 + mass channel.
The spectrum of 12 C2N complex in this region (Figure 2) comprises a single, broadened (ca.14 cm -1 full width half-maximum, FWHM) peak at approximately 2233 cm -1 .As discussed previously, 25 we are unable to resolve the N 2 O N=N stretch fundamental from the anti-phase CO stretch in this peak.Confirming this accidental degeneracy was one of the inspirations for this study as the isotope shift of 13 C should eliminate it.The N 2 O N=N stretch (labelled C) lies very weakly blue-shifted from the free N 2 O band (at 2223.5 cm -1 ) reflecting its weaker binding to the core structure.This is also indicated by the Au-N distance, determined by DFT calculations to be 3.026 Å.By contrast, the CO vibrational bands are strongly blue-shifted from the free CO stretch at 2143 cm -1 reflecting their "non-classical carbonyl" nature 27,28 arising from the cationic metal center and the resulting lack of -back-bonding.The very near linear Au(CO) 2 + core means that the in-phase CO stretch band has very weak oscillator strength (labelled B, near 2270 cm -1 ) and is not observed.
Isotopic substitution of a single 12 CO with a 13 CO leads to two major changes in the spectrum.
Firstly, both CO-stretching bands are notably red-shifted from their band positions in the 12 C2N spectrum reflecting the smaller reduced mass and its effect on the vibrational constant (  ∝ ).
Secondly, the reduced symmetry results in intensity in both the in-phase and anti-phase CO stretching bands which are observed at ~2253 cm -1 and ~2195 cm -1 , respectively.The spectrum of the 13 C2N complex, as expected, shows two clear bands, the N=N stretch (unchanged in all isotopologues) and the anti-phase CO band at 2183 cm -1 , ca. 45 cm -1 to the red of the equivalent band in 12 C2N.The in-phase stretching band is again essentially symmetry forbidden.
In all cases, the simulated spectra reproduce the experimental spectra remarkably well, leaving little doubt over the security of the assignments.This provides confidence in assigning the larger complexes.In a more general context, this ability to reliably move vibrational modes within mixed ligand complexes via isotope substitution offers hope in better resolving, and hence understanding, the spectra of other ion-molecule complexes. 28,29,38,39 b)ectra and fragmentation branching ratios of larger mixed-ligand complexes Figure 3 shows the comparison of experimental and simulated spectra for the four isotopologues of the Au( 12/13 CO) 3 (N 2 O) + complex.The structure is unambiguously assigned to a C 3v structure with trigonal planar Au(CO) 3 + core to which N 2 O binds along the C 3 axis via the terminal N atom. 25The addition of a third CO to the core brings the calculated CO binding energy down from 0.7 eV to 0.26 eV, still markedly higher than the 0.15 eV binding of the N 2 O ligand but accessible for IR photodissociation in the range of our spectra (0.265 -0.282 eV).
The spectra in Figure 3a-d  branching ratio no greater than 0.15, despite the fact that the binding energy of N 2 O to the rest of the complex (ca.0.15 eV) is the lowest of all.Instead, the dominant fragmentation pathway is CO loss (ca.0.26 eV binding energy) which, in these studies, can be resolved into 12 CO or 13 CO loss channels.The relative branching ratios for 12 CO vs 13 CO loss appears statistical, reflecting the number of each CO isotope in the parent complex.Hence the signal in the 13 CO loss channel (green in Figure 3f-h) in the 12 C2 13 CN complex is very close to twice that in the 12 CO loss channel (red) and vice versa in the 12 C 13 C2N complex.Importantly, the action spectra recorded in each of the single ligand loss channels are all essentially identical -pumping a N 2 O-centered mode does not lead to preferential N 2 O loss.Hence the fragmentation branching ratios are the same for every vibration pumped.This implies rapid and efficient IVR, with the photon energy quickly distributed around the complex rather than experiencing obvious bottlenecks at the weaker intermolecular complex bonds which might lead to rupture of that bond and loss of the chromophore ligand.
In addition to single ligand loss fragmentation channels, spectra are observed which correspond to the loss of multiple ligands, namely loss of both the N 2 O ligand and a CO (see Figure 3e-h).In fact, in each of the panels shown, the total signal observed in all -[N 2 O, CO] loss channels represents a comparable branching ratio to the simple N 2 O loss channel.It seems unlikely that any CO--N 2 O dimer species would be lost and thus we would have expected the energy threshold for the dual ligand loss to be significantly higher than for single ligand loss.Some caution is required in interpreting these dissociation pathways since we cannot exclude the possibility of multiple photon absorption.In our earlier study of Au(CO) n (N 2 O) m + which involved pure 12 CO, we performed a full infrared pulse energy study 25 and showed that single ligand loss occurs efficiently at the one-photon level but that -[N 2 O, CO] loss channel was dominated by two-photon absorption at the higher pulse energies available.The cost of 13 CO prevented such a study in this case but there is little reason to suggest there would be any difference here.
Our experiments cannot distinguish unambiguously between two photon absorption followed by fragmentation and two successive absorption-fragmentation events.However, small but clear differences are observed in the spectra recorded in some of the dual ligand loss channels compared to those recorded in single ligand loss channels.One clear difference lies in the relative intensities of the N 2 O and the CO-centred bands with the former appearing considerably more strongly in the spectra of the dual ligand loss (see Figure 3e).This is particularly true in the spectra of the 13 C2 12 CN and 13 C 12 C2N complexes, in which the CO bands in the 2150 -2175 cm -1 region are barely visible at all but also in the weak CO band in the 13 C3N complex.This suggests stepwise fragmentation with CO loss following the first photon absorption at the CO stretch.This would lead to significant core rearrangement to a linear Au(CO) 2 + structure and loss of the chromophore in the A1/A2 region.In all probability, it would also leave the fragment complex with significant internal energy leading to broadening of any subsequent absorption bands.There is still a small degree of fragmentation present in the 12 C3N spectrum (Figure 3e, pink trace), possibly the result of IR absorption at this perturbed Au(CO) 2 + structure but only for the anti-phase (nominally A2) CO stretch.Since none of the C2N complexes have absorption bands in the same spectral region (see Figure 2) no second photon could then be absorbed.Given the relative energy of the initial daughter complex compared to the putative ground state of the Au( 12/13 CO) 2 (N 2 O) + complex, a fertile avenue for future work could be to characterise the kinetic energy and rovibrational distribution of the ejected CO ligand. 40e fact that the N=N stretch mode at 2233 cm -1 is strong in all fragmentation channels for every complex implies that, although it is a strong chromophore, the N 2 O is not lost rapidly at the one photon level.This is consistent with the observation of more efficient CO loss in general.
Many of the same ideas introduced in the discussion of the spectra and dissociation dynamics of the C3N complexes apply to the C3N2 complexes (see Figure 4).Excellent agreement between experimental and simulated spectra ( The similarity of C3N2 complexes to C3N follows though into the fragmentation dynamics. Again the strongest branching ratios are observed for the CO loss channels although the effect is less marked in C3N2 given the second N 2 O ligand.To a good approximation, the relative single ligand fragmentation yields mirror the respective number of ligands present with the ratio of total CO loss to N 2 O approximately 3:2.This despite the calculated CO binding energy (at 0.26 eV) being a factor of two larger than that for N 2 O (0.12 eV).

c. Computational study of intramolecular energy flow
In an attempt to better understand the dissociation dynamics observed a nudged-elastic band optimization 41,42 was performed to estimate the minimum energy path for N 2 O and CO dissociation.The Gaussian 2016 program was again used for all electronic structure calculations. 36e result is shown in Figure 5  via heat transferred to vibrational motion perpendicular to, or in, the trigonal plane.The threshold for N 2 O loss (0.17 eV), is, however, markedly lower than that for CO loss (0.30 eV) and the preferential loss of CO observed experimentally remains hard to explain on energetic grounds.IR absorption by a localized vibrational mode heats the rest of the complex via anharmonic terms in the potential energy surface.Although the system undergoes substantial geometric changes upon dissociation, the early transfer of energy out of the initially excited zero-order bright state is driven by small anharmonic coupling terms.To analyse the coupling between modes, modes and how these can lead to competing dissociating pathways, whilst keeping the analysis tractable, we have partitioned modes into subsets corresponding to stretches and intermolecular ligand modes.Since the number of CO ligand modes is large, we selected those which are necessarily involved in the formation of the co-linear CO, C2N complex (approximately half of them).These also naturally describe the in-plane dissociation seen in Figure 5.To estimate the correlated space between modes, we proceeded to evaluate the PES.For every pair of modes possible, vectors along the diagonal of each quadrant were used to evaluate tens of ab-initio samples.We also generated hundreds of quasi-random (Sobol) samples for these same correlated spaces (see Figure S8 in supporting information).Mass-and frequency-scaled normal mode coordinates were used to approximately place all displacements into a similar domain.A cluster expansion of symmetryadapted polynomial functions was then fitted to the calculated data. 43Up to sixth order polynomials were used to fit the data with minimal error (typical average RMSD below 10 -5 eV).
To estimate the mode-pair correlation more accurately, the following root mean squared deviation (RMSD)   = 〈Δ 2  〉 , was numerically estimated using a Monte-Carlo approach such that: where k B is the Boltzmann constant, T the temperature and Z the canonical normalization function.
() is the model PES in the pair of coordinates space while   () corresponds to the model PES without any correlating (cross) terms.The integral measures the degree of correlation in the regions of phase-space accessed by the system at temperature T. This is done for each possible pair of modes and averaged for each set using a sampling temperature of 150 K which represents a reasonable estimate of the experimental conditions.Table 1 shows the average (relative) RMSD values of the various mode-pairs scaled to the N 2 O Ligand-N 2 O Ligand term (3.25x10 -2 eV) to illustrate their relative magnitude.This provides a more quantitative measure of the extent of coupling between the differing types of motion.

Table 1
Correlation matrix between different sets of modes.The correlation corresponds to the averaged RMSD difference between the correlated and uncorrelated cluster-expansion models fitted to the ab-initio data.Values are scaled to the N 2 O Ligand-N 2 O Ligand term (3.25x10 -2 eV) for ease of comparison.S3: around 50-70 cm −1 ) have lower frequencies than the corresponding CO ligand modes (modes 4-6e, 1a 2 and 3a 1 , 4a 1 ranging from 200 to 450 cm −1 ).
The CO ligand modes couple significantly to all modes (except the N 2 O stretch).Those CO ligand modes not included in the subset (orthogonal to the trigonal plane) are also likely to couple to the in-plane modes since these must be, at least in part, involved in the out-of-plane dissociation (blue) profile of Figure 5. Together, this trigonal framework forms a large anharmonically-coupled manifold plausibly 'funnelling' the available energy, such that stretch excitations may well preferentially heat the intermolecular CO ligand vibrations faster than the intermolecular N 2 O ligand vibrations.
Table 1 alone is not sufficient to explain the non-statistical experimental results and a closer examination is warranted.The ongoing construction of a model PES on which to perform quantum dynamical simulations, which will form the basis of a subsequent article, is briefly described here.
We use 'quasi-normal' coordinates which succinctly represent the competing dissociating channels, while continuing to form a basis for C 3v irreducible representations; this allowed us to accurately fit the Taylor model to the anharmonic basins, whilst permitting a further fit of the dissociating channels.This product-form model allows the application of wave packet methods 44 for which we can measure the picosecond energy transfer from high to low frequency modes, as well as the outgoing flux along the dissociating channels. IV.

ASSOCIATED CONTENT
The Supporting Information is available free of charge at -[insert link], including: geometric structure files for all complexes studied; normal mode analyses; example fragmentation mass spectra; dissociation branching ratios; further details on calculated dynamics and vibrational mode coupling rsc.li/pccp PCCP Physical Chemistry Chemical Physics rsc.li/pccpISSN 1463-9076 PAPER H.-P. Loock et al.Determination of the thermal, oxidative and photochemical degradation rates of scintillator liquid by fluorescence EEM spectroscopy Introduction CO/ 13 CO ligands, complete separation proved impossible without unacceptable transmission losses.However, the mass resolving power of the detection reflection time of flight mass spectrometer was sufficient to determine the fragmentation channels unambiguously.The supporting information includes the mass spectrum of Au( 13/12 CO) n (N 2 O) + species along with photofragment mass spectra collected at   = 2157 cm −1 , 2207 cm −1 , and 2230 cm −1 illustrating that photofragmentation occurs by loss of CO, of N 2 O or of both (Figures S2-

Figure 1 :
Figure 1: Time of flight mass spectrum of Au( 13 CO) n (N 2 O) m + complexes produced by laser ablation of a gold target in presence of a carrier gas comprising 1%

Figure 2
Figure2shows a comparison of the infrared spectra for the isomer-selected Au( 12/13 CO) 2 (N 2 O) +

Page 10 of 32 Figure 2 :
Figure 2: Comparison of experimental and simulated infrared action spectra of figure 2. Only two bands are IR active in the spectrum of the 12 C3N and 13 C3N complexes, the

Figure 3 a
Figure 3 a)-d) Comparison of experimental and simulated infrared action spectra of Figure 4a) lead to clear and unambiguous spectral assignments once more.The trigonal bipyramidal (D 3h ) structure exhibits very similar spectral bands to the C3N complexes.The Au(CO) 3 + core has identical vibrations and only the anti-phase N 2 O stretching band is infrared allowed.The additional N 2 O ligand results in enhance intensity of the 2233 cm -1 N=N stretch band relative to the CO modes.

Page 18 of 32
Physical Chemistry Chemical Physics Physical Chemistry Chemical Physics Accepted Manuscript Open Access Article.Published on 20 May 2024.Downloaded on 5/27/2024 9:16:39 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Figure 4 a
Figure 4 a)-d) Comparison of experimental and simulated infrared action spectra of with full details provided in supplementary information.The abscissa coordinate is the distance between the gold atom and the centre of mass of the leaving ligand.Three cuts across the potential energy surface (PES) are shown, relative to the minimum energy structure, with figures of the path enclosed in the same colour as the cut.The lowest (black) potential corresponds to loss of the N 2 O ligand, while the red and blue cuts correspond to different pathways leading to CO loss; The CO-p (red) curve corresponds to CO leaving within the plane of the CO core and the CO-o (blue) curve a path orthogonal to the trigonal plane.The two are similar except in the close-range region in which CO-o is steeper.CO cleavage can clearly occur

Page 22 of 32 Figure 5
Figure 5 Minimum energy paths for the dissociation of CO and N2O ligands.CO-p (red) 13 CO substitution has been used to better understand the structure and spectroscopy of mixed ligand Au + (CO) n (N 2 O) m ion-molecule complexes.The agreement between experimental infrared action spectra and simulated vibrational spectra of calculated low energy structures is good and provides for unambiguous identification of the structures generated.Similar action spectra are observed in all single-ligand loss channels indicating rapid intramolecular vibrational redistribution following infrared absorption.Only N 2 O loss is observed for Au + (CO) 2 (N 2 O) complexes, reflecting the strong binding of the Au + (CO) 2 core structure.No clear isotopologue effects were observed in the dissociation yields with12 CO vs13 CO loss in stoichiometric ratios within experimental uncertainty.In Au + (CO) 3 (N 2 O) 1,2 complexes, however, a clear preference for CO loss is once more observed over N 2 O loss despite the binding energy for the latter being considerably smaller.This non-statistical fragmentation has been investigated computationally both by computing vibrational mode couplings on an ab initio potential surface and with nudged elastic band dissociative potential energy curves.The former yield insight into potential bottlenecks in the energy flow around the complex following vibrational excitation, with coupling between the CO---Au + intermolecular modes and the N 2 O stretch particularly weak.Weak coupling between the CO ligand modes and the N 2 O ligand modes together with the larger phase space of the former, suggest that energy may be effectively trapped in the Au + (CO) n core leading to more efficient CO loss than N 2 O when energetically allowed.

Table 1
shows that the stretching modes couple comparably well with their respective ligand modes.However, CO stretching also couples strongly to N 2 O ligand modes and conversely, N 2 O stretching modes weakly couples with CO ligand modes.This appears contrary to the experimental observation that pumping either CO or N 2 O stretches leads to effective and similar CO fragmentation.However, time-dependent perturbation theory suggests population transfer depends not only on coupling, but inversely on frequency difference.In this case, N 2 O ligands may consequently take longer to become excited; N 2 O ligand dissociating modes (modes 1a, 2a, see Table