Two algorithms for calculating overlaps between CIS (or TDDFT) type excited state wave functions are presented, one based on expansion of overlap determinants into level 2 minors (OL2M) and the other based on an expansion of the wave functions into natural transition orbitals (ONTO). Both are significantly faster than previously available algorithms, with the ONTO algorithm reducing the cost of a single overlap element calculation by a factor of the square of the number of occupied orbitals in the system, resulting in orders of magnitude faster calculations for large systems and significantly increasing the size of systems for which TDDFT based nonadiabatic dynamics simulations can be performed. The OL2M algorithm is significantly slower for a single overlap matrix element, but becomes preferred when overlaps between large numbers of states are required. Additionally, we test the accuracy of approximate overlaps calculated using truncated wave functions and show that truncation can lead to very large errors in the overlaps. Lastly, we provide examples of applications for wave function overlaps outside the context of nonadiabatic dynamics.

In order to test metal diketonates as potential acceptors of bifurcated halogen bonds, the series of acetylacetonates (acac) of divalent cations Cu­(acac)2 (1), Pd­(acac)2 (2), VO­(acac)2 (3), Ni­(acac)2(H2O)2 (4), Co­(acac)2(H2O)2 (5), and Zn (acac)2(H2O) (6) were cocrystallized with 1,4-diiodotetrafluorobenzene (tfib) and 1,4-dibromotetrafluorobenzene (tfbb) as halogen donors. This has yielded a series of 10 cocrystals, tfib having formed cocrystals with all six acceptors and tfbb with all except for 4 and 5. In eight cocrystals a pair of acac oxygen atoms acts as a bifurcated halogen bond acceptor, the bond being symmetric in cocrystals of 1 and 2 and asymmetric in cocrystals of 3 and 6. The only cocrystals in which a halogen bond was formed with alternative acceptor sites were cocrystals of tfib with 4 and 5, where coordinated water molecules form hydrogen bonds with all available acac oxygen atoms, leaving only the water molecules themselves as halogen bond acceptors. The favorability of the bifurcated halogen bond was also confirmed by QM computations, which have shown the bifurcated bonds to be the most favorable interactions in vacuo, with bond energies in the range of 29–37 kJ mol–1 for tfib and 20–25 kJ mol–1 for tfbb. This also reflects on the thermal stability of the cocrystals of 1–3 (which do not contain coordinated water) with tfib, which melt/decompose between ca. 180 and 220 °C.

To investigate influences on the topicity of perfluorinated halobenzenes as halogen bond (XB) donors in the solid state, we have conducted a database survey and prepared 18 novel cocrystals of potentially ditopic (13ditfb, 14ditfb) and tritopic (135titfb) XB donors with 15 monotopic pyridines. 135titfb shows high tendency to be mono-or ditopic, but with strong bases it can act as a tritopic XB donor. DFT calculations have shown that binding of a single acceptor molecule on one of the iodine atoms of the XB donor reduces the ESP max on the remaining iodine atoms and dramatically decreases their potential for forming further halogen bonds, which explains both the high occurrence of crystal structures where the donors do not achieve their maximal topicity and the observed differences in halogen bond lengths. Despite the fact that this effect increases with the basicity of the acceptor, when the increase of halogen bond energy due to the basicity of the acceptor compensates its decrease due to the reduction of the acidity of the donor, it enables strong bases to form cocrystals in which a potentially polytopic XB donor achieves its maximal topicity.

Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photochemical formation of carbenes from diazoalkanes has not been proposed. Synergy of experiments and computations demonstrates the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazoalkanes, excitation to S1 results in nonreactive internal conversion to S0. On the contrary, excitation to higher-lying singlet states (Sn, n > 1), drives the reaction toward a different segment of the S1/S0 conical intersection seam and results in nitrogen elimination and formation of carbenes.

A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the 1 B 2u (ππ*) state. The simulated spectra and the asymmetry map are compared to the experimental data, and a very good agreement was obtained without applying any energy-dependent rescaling or broadening. An interesting side result of this work is the finding that the signature of the 1 A u (nπ*) state is indistinguishable from that of the 1 B 3u (nπ*) state in the time-resolved photoelectron spectrum. By locating four symmetrically equivalent minima on the lowest-excited (S1) adiabatic potential energy surface of pyrazine, we revealed the strong vibronic coupling of the 1 A u (nπ*) and 1 B 3u (nπ*) states near the S1 ← S0 band origin.

We explore the halogen bond acceptor potential of the morpholinyl oxygen atom in the synthesis of cocrystals involving organic and metal−organic units, by using N-aminomorpholine either as a potential halogen bond acceptor or as a reagent to insert a morpholine moiety into larger organic and copper(II)-based metal−organic building blocks. Challenged against four well-known halogen bond donor molecules differing in binding geometry and composition, these three morpholinecontaining units have yielded a total of seven previously not reported cocrystals, of which six (86%) exhibited the formation of I•••O or Br•••O halogen bonds to the morpholinyl oxygen atom. The results illustrate the possibility to systematically insert and use a morpholine group as an efficient halogen bond acceptor into organic and metal−organic structures, thus enabling cocrystal formation.

A series of 15 salts and cocrystals of an asymmetric aromatic–aliphatic dicarboxylic diacid, N-(2-carboxyphenyl)­glycine, and pyridine derivatives was synthesized in order to study whether there is a preference of pyridine binding to either of the carboxylic groups. In nine structures the pyridine was bonded to the aliphatic group, in three to both carboxylic groups, and in three to the aromatic group alone. The preference of pyridine derivatives to bond to the aliphatic group correlates with the more positive electrostatic potential on the aliphatic carboxyl hydrogen atom. The occurrence of proton transfer within the structures was found to follow the basicity of the pyridine derivative, four of the least basic pyridines forming cocrystals, two intermediate bases solids with proton disorder and the more basic pyridine derivatives salts. Proton transfer had a considerable effect on the crystal packing, cocrystals mostly comprising discrete molecular complexes, and salt chains of monoanions.

The photochemically induced ring-opening isomerization reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene is a textbook example of a pericyclic reaction and has been amply investigated with advanced spectroscopic techniques. The main open question has been the identification of the single reactive state which drives the process. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest lying bright state, followed by a passage through a conical intersection to the lowest lying doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. Here, in a joint experimental and computational effort, we demonstrate that the evolution of the excitation−deexcitation process is much more complex than that usually described. In particular, we show that an initially highlying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction.