Computational Versus Experimental Spectroscopy …
169
4 Transient Species: The Case of Nickel(IV) Tris-µ-Oxido
The identification of transient intermediates, i.e., species with short lifetimes,
relies heavily on time-resolved spectroscopy, especially where such species are not
amenable to trapping in crystalline form. In contrast to photochemically generated
intermediates (such as those of photosystem II, in which a substantial population of
an intermediate is generated by the actinic pulse), in thermal reactions the steadystate population depends on the reaction kinetics, and is controllable only to a limited
extent by temperature and solvent.
In thermal reactions, the elucidation of molecular structures from spectroscopy is
essential and relies primarily on precedent: we associate spectral features observed
with those of species that are known, and we use analogy to guide us. However, what
if the structure has no real precedent, or is suspected to be similar to a hypothesized
but never observed structure? In such a situation, we are faced with a challenge to
recognize the new species, and we turn to experience and bring to bear as many
appropriate spectroscopic techniques as we can to gather the evidence needed to
unmask the structure and character of the intermediate. For these cases, theory provides us with a powerful tool through its ability to test our hypothesized structures
both for thermodynamic feasibility and for prediction of spectroscopic properties. In
this section, we aim to show, however, that rather than being a separate aspect of such
a study, theory is integral to the process. The example taken, a recently identified
nickel intermediate [40], is of course anecdotal and our aim is not to summarize the
findings but rather to focus on the approach taken and the order in which experiments
were actually done.
The study began with the observation of a pronounced but short-lived (seconds)
visible absorption band (Fig. 6) when near-stoichiometric NaOCl(aq) was added to
methanol solutions of the known complex [(Ni
II ) 2 (Cl) 3 (tmtacn) 2 ]
2+ (where tmtacn is
1,4,7-trimethyl-1,4,7-triazacyclononane). The species was longer lived in acetonitrile
but the scatter caused by precipitated NaCl in it, more or less canceled the benefit in
regard to spectroscopic features.
Our anecdote begins at this point, with the immediate curiosity as to what species
had been observed, i.e., what is the molecular structure of the visible chromophore.
The obvious choice of techniques, at least to us, was to use ESI-MS and Raman
spectroscopy. ESI-MS relies on the species being charged and sufficiently stable to
remain intact during generation of the electrospray, while the use of Raman spectroscopy relies on signal enhancement (resonance enhancement) that can occur when
the wavelength of excitation is coincident with the visible absorption band.
ESI-MS was useful to show that the dinuclear structure of
[(Ni
II ) 2 (Cl) 3 (tmtacn) 2 ]
2+ is dynamic, i.e., we should not view the complex in
solution as being kinetically inert to exchange of the chlorido ligands. That is, the
ESI-MS spectra obtained just after adding NaOCl (the active component of bleach)
indicated a new doubly charged ion with [Ni 2 (O) 3 (tmtacn) 2 ]
2+ as chemical formula.
It showed the expected isotope sensitivity when NaOCl equilibrated (which occurs
essentially instantaneously upon mixing) in H
18
2 O. Of course, ESI-MS can say little
169
4 Transient Species: The Case of Nickel(IV) Tris-µ-Oxido
The identification of transient intermediates, i.e., species with short lifetimes,
relies heavily on time-resolved spectroscopy, especially where such species are not
amenable to trapping in crystalline form. In contrast to photochemically generated
intermediates (such as those of photosystem II, in which a substantial population of
an intermediate is generated by the actinic pulse), in thermal reactions the steadystate population depends on the reaction kinetics, and is controllable only to a limited
extent by temperature and solvent.
In thermal reactions, the elucidation of molecular structures from spectroscopy is
essential and relies primarily on precedent: we associate spectral features observed
with those of species that are known, and we use analogy to guide us. However, what
if the structure has no real precedent, or is suspected to be similar to a hypothesized
but never observed structure? In such a situation, we are faced with a challenge to
recognize the new species, and we turn to experience and bring to bear as many
appropriate spectroscopic techniques as we can to gather the evidence needed to
unmask the structure and character of the intermediate. For these cases, theory provides us with a powerful tool through its ability to test our hypothesized structures
both for thermodynamic feasibility and for prediction of spectroscopic properties. In
this section, we aim to show, however, that rather than being a separate aspect of such
a study, theory is integral to the process. The example taken, a recently identified
nickel intermediate [40], is of course anecdotal and our aim is not to summarize the
findings but rather to focus on the approach taken and the order in which experiments
were actually done.
The study began with the observation of a pronounced but short-lived (seconds)
visible absorption band (Fig. 6) when near-stoichiometric NaOCl(aq) was added to
methanol solutions of the known complex [(Ni
II ) 2 (Cl) 3 (tmtacn) 2 ]
2+ (where tmtacn is
1,4,7-trimethyl-1,4,7-triazacyclononane). The species was longer lived in acetonitrile
but the scatter caused by precipitated NaCl in it, more or less canceled the benefit in
regard to spectroscopic features.
Our anecdote begins at this point, with the immediate curiosity as to what species
had been observed, i.e., what is the molecular structure of the visible chromophore.
The obvious choice of techniques, at least to us, was to use ESI-MS and Raman
spectroscopy. ESI-MS relies on the species being charged and sufficiently stable to
remain intact during generation of the electrospray, while the use of Raman spectroscopy relies on signal enhancement (resonance enhancement) that can occur when
the wavelength of excitation is coincident with the visible absorption band.
ESI-MS was useful to show that the dinuclear structure of
[(Ni
II ) 2 (Cl) 3 (tmtacn) 2 ]
2+ is dynamic, i.e., we should not view the complex in
solution as being kinetically inert to exchange of the chlorido ligands. That is, the
ESI-MS spectra obtained just after adding NaOCl (the active component of bleach)
indicated a new doubly charged ion with [Ni 2 (O) 3 (tmtacn) 2 ]
2+ as chemical formula.
It showed the expected isotope sensitivity when NaOCl equilibrated (which occurs
essentially instantaneously upon mixing) in H
18
2 O. Of course, ESI-MS can say little