Pulsed EPR Research Articles

Characterization of low sodium type II silicon clathrate film spin dynamics

Type II Si clathrate is a Si-based, crystalline alternative to diamond silicon with interesting optoelectronic properties. Here, a pulsed electron paramagnetic resonance study of the spin dynamics of sodium-doped, type II NaxSi136 silicon clathrate films is reported. Focusing on the hyperfine lines of isolated Na atoms, the temperature dependence of the electron spin dynamics is examined from 6 to 25 K. The measurements exhibit multi-exponential decay, indicating multiple spin relaxation rates in the system. As expected, spin relaxation time (T1) increases rapidly with decreasing temperature, reaching ∼300 μs at 6.4 K. The phase memory (TM) shows less temperature dependence with a value of ∼3 μs at the same temperature. The temperature dependence of T1 exhibits Arrhenius behavior in the measurement range consistent with an Orbach pathway. There are strong similarities to the spin behavior of other defect donors in diamond silicon. The results provide insights into the potential of Si clathrates for spin-based applications.

The contribution of methyl groups to electron spin decoherence of nitroxides in glassy matrices.

Long electron spin coherence lifetimes are crucial for high sensitivity and resolution in many pulse electron paramagnetic resonance (EPR) experiments aimed at measuring hyperfine and dipolar couplings, as well as in potential quantum sensing applications of molecular spin qubits. In immobilized systems, methyl groups contribute significantly to electron spin decoherence as a result of methyl torsional quantum tunneling. We examine the electron spin decoherence dynamics of the nitroxide radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) in both a methyl-free solvent and a methyl-containing solvent at cryogenic temperature. We model nitroxide and solvent methyl effects on decoherence using cluster correlation expansion (CCE) simulations extended to include methyl tunneling and compare the calculations to experimental data. We show that by using the methyl tunneling frequency as a fit parameter, experimental Hahn echo decays can be reproduced fairly well, allowing structural properties to be investigated in silico. In addition, we examine the Hahn echo of a hypothetical system with an unpaired electron and a single methyl to determine the effect of geometric configuration on methyl-driven electron spin decoherence. The simulations show that a methyl group contributes the most to electron spin decoherence if it is located between 2.5 and 6-7Å from the electron spin, with its orientation being of secondary importance.

Stable Mono‐radical and Triplet Diradicals Based on Allylic Radical‐Embedded All‐benzenoid Polycyclic Hydrocarbons

High‐spin organic radicals are notable for their unique optical, electronic, and magnetic properties, but synthesizing stable high‐spin systems is challenging due to their inherent reactivity. This study presents a novel strategy for designing stable high‐spin polycyclic hydrocarbons (PHs) by incorporating allylic radical into fused aromatic benzenoid rings. To enhance stability, large steric hindrance groups with a synergistic captodative effect were added to the allylic radical centers. This approach was applied to synthesize derivatives of two open‐shell all‐benzenoid PHs: benzo[fg]tetracene (1) and dibenzo[fg,lm]heptacene (2). Both compounds exhibited excellent stability, with diradical 2 showing a half‐life of up to 17.2 days under ambient conditions. Bond length analysis and theoretical calculations suggest that 1 and 2 predominantly feature Clar’s aromatic sextet structures with embedded allylic radicals. Compound 2 was confirmed to have a triplet ground state through DFT calculations and experimental methods, including pulse EPR spectroscopy and SQUID measurements. This work introduces a new design strategy for stable high‐spin hydrocarbons, paving the way for future developments in high‐spin organic materials.

Stable Mono-radical and Triplet Diradicals Based on Allylic Radical-Embedded All-benzenoid Polycyclic Hydrocarbons.

High-spin organic radicals are notable for their unique optical, electronic, and magnetic properties, but synthesizing stable high-spin systems is challenging due to their inherent reactivity. This study presents a novel strategy for designing stable high-spin polycyclic hydrocarbons (PHs) by incorporating allylic radical into fused aromatic benzenoid rings. To enhance stability, large steric hindrance groups with a synergistic captodative effect were added to the allylic radical centers. This approach was applied to synthesize derivatives of two open-shell all-benzenoid PHs: benzo[fg]tetracene (1) and dibenzo[fg,lm]heptacene (2). Both compounds exhibited excellent stability, with diradical 2 showing a half-life of up to 17.2 days under ambient conditions. Bond length analysis and theoretical calculations suggest that 1 and 2 predominantly feature Clar's aromatic sextet structures with embedded allylic radicals. Compound 2 was confirmed to have a triplet ground state through DFT calculations and experimental methods, including pulse EPR spectroscopy and SQUID measurements. This work introduces a new design strategy for stable high-spin hydrocarbons, paving the way for future developments in high-spin organic materials.

I2BODIPY as a new photoswitchable spin label for light-induced pulsed EPR dipolar spectroscopy exploiting magnetophotoselection.

Electron paramagnetic resonance (EPR) pulsed dipolar spectroscopy (PDS) using triplet states of organic molecules is a growing area of research due to the favourable properties that these transient states may afford over stable spin centers, such as switchability, increased signal intensity when the triplet is formed in a non-Boltzmann distribution and the triplet signal is used for detection, and high orientation selection, when the triplet signal is probed by microwave pulses. This arises due to the large spectral width at low fields, a result of the large zero field splitting, and limited bandwidth of microwave pulses used. Here we propose the triplet state of a substituted BODIPY moiety as a spin label in light induced PDS, coupled to a nitroxide, in a model peptide with a rigid structure. Orientation selection allows information on the relative position of the centres of the two labels to be obtained with respect to the nitroxide reference frame. Additionally, magnetophotoselection effects are employed to introduce optical selection and additional constraints for the determination of the relative orientation of the spin labels considering the reference frame of the triplet state.

Pulsed EPR Methods in the Angstrom to Nanometre Scale Shed Light on the Conformational Flexibility of a Fluoride Riboswitch.

Riboswitches control gene regulation upon external stimuli such as environmental factors or ligand binding. The fluoride sensing riboswitch from Thermotoga petrophila is a complex regulatory RNA proposed to be involved in resistance to F- cytotoxicity. The details of structure and dynamics underpinning the regulatory mechanism are currently debated. Here we demonstrate that a combination of pulsed electron paramagnetic resonance (ESR/EPR) spectroscopies, detecting distances in the angstrom to nanometre range, can probe distinct regions of conformational flexibility in this riboswitch. PELDOR (pulsed electron-electron double resonance) revealed a similar preorganisation of the sensing domain in three forms, i.e. the free aptamer, the Mg2+-bound apo, and the F--bound holo form. 19F ENDOR (electron-nuclear double resonance) was used to investigate the active site structure of the F--bound holo form. Distance distributions without a priori structural information were compared with in silico modelling of spin label conformations based on the crystal structure. While PELDOR, probing the periphery of the RNA fold, revealed conformational flexibility of the RNA backbone, ENDOR indicated low structural heterogeneity at the ligand binding site. Overall, the combination of PELDOR and ENDOR with sub-angstrom precision gave insight into structural organisation and flexibility of a riboswitch, not easily attainable by other biophysical techniques.

Eustigmatophyte model of red-shifted chlorophyll a absorption in light-harvesting complexes

Photosynthetic organisms harvest light for energy. Some eukaryotic algae have specialized in harvesting far-red light by tuning chlorophyll a absorption through a mechanism still to be elucidated. Here, we combined optically detected magnetic resonance and pulsed electron paramagnetic resonance measurements on red-adapted light-harvesting complexes, rVCP, isolated from the freshwater eustigmatophyte alga Trachydiscus minutus to identify the location of the pigments responsible for this remarkable adaptation. The pigments have been found to belong to an excitonic cluster of chlorophylls a at the core of the complex, close to the central carotenoids in L1/L2 sites. A pair of structural features of the Chl a403/a603 binding site, namely the histidine-to-asparagine substitution in the magnesium-ligation residue and the small size of the amino acid at the i-4 position, resulting in a [A/G]xxxN motif, are proposed to be the origin of this trait. Phylogenetic analysis of various eukaryotic red antennae identified several potential LHCs that could share this tuning mechanism. This knowledge of the red light acclimation mechanism in algae is a step towards rational design of algal strains in order to enhance light capture and efficiency in large-scale biotechnology applications.

Investigation of slow magnetic relaxation in a series of 1D polymeric cyclobutane-1,1-dicarboxylates based on LnIIIVIV2 units (LnIII = Tb, Dy, Ho, Er, Tm, Yb): rare examples of VIV-4f single-molecule magnets.

The reactions of VOSO4·3H2O with Na2(cbdc) (cbdc2- - dianion of cyclobutane-1,1-dicarboxylic acid) and lanthanide(III) nitrates taken in a molar ratio of 1 : 2 : 1 were found to yield a series of isostructural heterometallic compounds [NaLn(VO)2(cbdc)4(H2O)10]n (1Ln, Ln = Tb, Dy, Ho, Er, Tm, Yb). These compounds are constructed from trinuclear anionic units [Ln(VO)2(cbdc)4(H2O)8]- ({LnV2}-) linked by Na+ ions into 1D polymeric chains. The crystal structures of 1Dy and 1Er were determined by single-crystal X-ray diffraction (XRD), and their isostructurality with 1Tb, 1Ho, 1Tm, and 1Yb was proved by powder X-ray diffraction (PXRD). According to alternating current (ac) magnetic susceptibility measurements, 1Dy, 1Er, and 1Yb exhibited field-induced slow relaxation of magnetization. Compound 1Er is the first representative of ErIII-VIV single-molecule magnets. Measuring the temperature dependences of the phase memory time (Tm) for 1Dy and 1Yb using pulsed EPR spectroscopy allowed us to observe the phenomenon of phase relaxation enhancement (PRE) at temperatures below 30 K. In future, this phenomenon may contribute to the evaluation of relaxation times of the lanthanide ions.

Proton-Coupled Electron Transfer between a Pendant Thiol and a Ferrous Dioxygen Adduct.

The mechanism of thiol oxidation by O2, as catalyzed by ferrous porphyrins, is investigated by trapping intermediates of the transformation at low temperatures and subsequently characterizing them using continuous wave and pulsed electron paramagnetic resonance and resonance Raman spectroscopy. A Fe(III)-O2•- species is initially formed in an iron porphyrin with a pendant thiol functional group, which undergoes proton-coupled electron transfer (PCET) (not HAT) to form an Fe(III)-OOH species. Following O-O bond homolysis, this forms a Fe(IV)═O species with concomitant oxidation of the thiol to an RSO3H group.

Re-examination of the intumescence mechanism of fire retarded PP with APP/pentaerythritol/zeolite-4A using advanced spectroscopic techniques

The mixture of ammonium polyphosphate (APP) and pentaerythritol (PER) is a very efficient flame retardant (FR) intumescent system suitable for polyolefins such as polypropylene (PP). The mechanisms of intumescence of this FR system in this polymer was investigated using different spectroscopic techniques including continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy and solid state nuclear magnetic resonance (NMR) technique. In this work, the intumescence mechanism of PP/APP/PER formulations with and without 4A is revisited. The intumescent system was in-depth investigated using NMR, CW EPR and pulsed EPR. The CW EPR technique confirmed that free radicals are mainly generated during the intumescence of the system between 250 and 350 °C. Thanks to the pulsed EPR and solid state NMR, it was evidenced that a key structural shift from a predominantly carbonaceous residue to a predominantly phosphorated residue. Besides, it was also evidenced that zeolite 4A totally collapses during extrusion of PP/APP/PER formulations reacting with APP to generate aluminophosphates. Then, silicophosphates are generated between 350 and 400 °C. Both alumino- and silicophosphates contribute to protect aromatic structures in the residue at high temperatures.

A Paramagnetic Nickel–Zinc Hydride Complex

AbstractReaction of a molecular zinc–hydride [{(ArNCMe)2CH}ZnH] (Ar=2,6‐di‐isopropylphenyl) with 0.5 equiv. of [Ni(CO)Cp]2 led to the isolation of a nickel–zinc hydride complex containing a bridging 3‐centre,2‐electron Ni−H−Zn interaction. This species has been characterized in the solid‐state by single crystal X‐ray diffraction. DFT calculations are consistent with its formulation as a σ‐complex derived from coordination of the zinc–hydride to a paramagnetic nickel(I) fragment. Continuous‐wave and pulse EPR experiments suggest that this species is labile in solution. Further experiments show that in the presence of catalytic quantities of nickel(I) precursors, zinc–hydride bonds can undergo either H/D‐exchange with D2 or dehydrocoupling to form Zn−Zn bonds. In combination, the data support the activation and functionalisation of zinc–hydride bonds at nickel(I) centres.

Dynamic basis of lipopolysaccharide export by LptB2FGC

Lipopolysaccharides (LPS) confer resistance against harsh conditions, including antibiotics, in Gram-negative bacteria. The lipopolysaccharide transport (Lpt) complex, consisting of seven proteins (A-G), exports LPS across the cellular envelope. LptB2FG forms an ATP-binding cassette transporter that transfers LPS to LptC. How LptB2FG couples ATP binding and hydrolysis with LPS transport to LptC remains unclear. We observed the conformational heterogeneity of LptB2FG and LptB2FGC in micelles and/or proteoliposomes using pulsed dipolar electron spin resonance spectroscopy. Additionally, we monitored LPS binding and release using laser-induced liquid bead ion desorption mass spectrometry. The β-jellyroll domain of LptF stably interacts with the LptG and LptC β-jellyrolls in both the apo and vanadate-trapped states. ATP binding at the cytoplasmic side is allosterically coupled to the selective opening of the periplasmic LptF β-jellyroll domain. In LptB2FG, ATP binding closes the nucleotide binding domains, causing a collapse of the first lateral gate as observed in structures. However, the second lateral gate, which forms the putative entry site for LPS, exhibits a heterogeneous conformation. LptC binding limits the flexibility of this gate to two conformations, likely representing the helix of LptC as either released from or inserted into the transmembrane domains. Our results reveal the regulation of the LPS entry gate through the dynamic behavior of the LptC transmembrane helix, while its β-jellyroll domain is anchored in the periplasm. This, combined with long-range ATP-dependent allosteric gating of the LptF β-jellyroll domain, may ensure efficient and unidirectional transport of LPS across the periplasm.

Dynamic basis of lipopolysaccharide export by LptB2FGC.

Lipopolysaccharides (LPS) confer resistance against harsh conditions, including antibiotics, in Gram-negative bacteria. The lipopolysaccharide transport (Lpt) complex, consisting of seven proteins (A-G), exports LPS across the cellular envelope. LptB2FG forms an ATP-binding cassette transporter that transfers LPS to LptC. How LptB2FG couples ATP binding and hydrolysis with LPS transport to LptC remains unclear. We observed the conformational heterogeneity of LptB2FG and LptB2FGC in micelles and/or proteoliposomes using pulsed dipolar electron spin resonance spectroscopy. Additionally, we monitored LPS binding and release using laser-induced liquid bead ion desorption mass spectrometry. The β-jellyroll domain of LptF stably interacts with the LptG and LptC β-jellyrolls in both the apo and vanadate-trapped states. ATP binding at the cytoplasmic side is allosterically coupled to the selective opening of the periplasmic LptF β-jellyroll domain. In LptB2FG, ATP binding closes the nucleotide binding domains, causing a collapse of the first lateral gate as observed in structures. However, the second lateral gate, which forms the putative entry site for LPS, exhibits a heterogeneous conformation. LptC binding limits the flexibility of this gate to two conformations, likely representing the helix of LptC as either released from or inserted into the transmembrane domains. Our results reveal the regulation of the LPS entry gate through the dynamic behavior of the LptC transmembrane helix, while its β-jellyroll domain is anchored in the periplasm. This, combined with long-range ATP-dependent allosteric gating of the LptF β-jellyroll domain, may ensure efficient and unidirectional transport of LPS across the periplasm.

Electron Spin Dynamics of the Intersystem Crossing in Aminoanthraquinone Derivatives: The Spectral Telltale of Short Triplet Excited States.

We studied the excited state dynamics of two bis-amino substituted anthraquinone (AQ) derivatives, with absorption in the visible spectral region, which results from the attachment of a electron-donating group to the electron-deficient AQ chromophore. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190-320 ps, and nanosecond transient absorption spectra demonstrated an unusually short triplet state lifetime (2.06-5.43 μs) for the two AQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at a longer delay time after laser flash. Spectral simulations show faster decay of the Ty sublevel than the other two sublevels (τx = 15.0 μs, τy = 1.50 μs, τz = 15.0 μs); theoretical computation predicts initial overpopulation of the Ty sublevel, and rationalizes the short T1 state lifetime and the ESP inversion. Theoretical computations taking into account the electron-vibrational coupling, i.e., the Herzberg-Teller effect, successfully rationalize the TREPR experimental observations.

Observation of magnetic dimers in the diluted triangular-lattice spin liquid candidate NaYbO2

The triangular-lattice spin liquid (QSL) candidate NaYbO2 is magnetically diluted by Lu doping, forming homogeneous solid solutions NaYb1-xLuxO2 (0≤ x<1.0). A combination of structural, magnetic, and pulsed high-field electron spin resonance analysis shows that the lattice parameters and the Curie-Weiss temperature θCW vary almost linearly with x, and the crystal electric field environment of the Yb/Lu ions persists during the Lu doping. The statistical distribution analysis of clusters and the deduced linear θCW ∼ x relation indicate that the nearest neighbor exchange J does not change with magnetic dilution. Interestingly, with the magnetization measurement at 0.39 K, we have observed Seff = 1/2 antiferromagnetic Yb-Yb dimers in NaYb0.02Lu0.98O2, which is manifested as a magnetization step at μ0Hc = 2.68 T due to the transition between singlet and triplet states. Thus, the intradimer interaction is directly determined as J/kB = −5.15 K. Surprisingly, this value is close to the interaction of undoped NaYbO2, supporting the theoretical analysis of clusters. Our study provides a method of estimating the magnetic interaction of QSL materials through magnetic dilution. The statistical distribution analysis of magnetic clusters suggests that through a small amount of Lu doping (x < 0.5), the QSL state in NaYbO2 can continue to be maintained.

CaY@C2n: Exploring Molecular Qubits with Ca-Y Metal-Metal Bonds.

Metal-metal bonding is crucial in chemistry for advancing our understanding of the fundamental aspects of chemical bonds. Metal-metal bonds based on alkaline-earth (Ae) elements, especially the heavier Ae elements (Ca, Sr, and Ba), are rarely reported due to their high electropositivity. Herein, we report two heteronuclear di-EMFs CaY@Cs(6)-C82 and CaY@C2v(5)-C80, which contain unprecedented single-electron Ca-Y metal-metal bonds. These compounds were characterized by single-crystal X-ray crystallography, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations. The crystallographic study of CaY@Cs(6)-C82 shows that Ca and Y are successfully encapsulated into the carbon cage with a Ca-Y distance of 3.691 Å. The CW-EPR study of both CaY@Cs(6)-C82 and CaY@C2v(5)-C80 exhibits a doublet, suggesting the presence of an unpaired electron located between Ca and Y. The combined experimental and theoretical results confirm the presence of a Ca-Y single-electron metal-metal bond with substantial covalent interaction, attributed to significant overlap between the 4s4p orbitals of Ca and the 5s5p4d orbitals of Y. Furthermore, pulse EPR spectroscopy was used to investigate the quantum coherence of the electron spin within this bond. The unpaired electron, characterized by its s orbital nature, is effectively protected by the carbon cage, resulting in efficient suppression of both spin-lattice relaxation and decoherence. CaY@Cs(6)-C82 behaves as an electron spin qubit, displaying a maximum decoherence time of 7.74 μs at 40 K. This study reveals an unprecedented Ae-rare-earth metal-metal bond stabilized by the fullerene cages and elucidates the molecular qubit properties stemming from their unique bonding character, highlighting their potential in quantum information processing applications.

A comparison of pulse and CW EPR [formula omitted]-relaxation measurements of an inhomogeneously broadened nitroxide spin probe undergoing Heisenberg spin exchange 2. The intercept discrepancy

Experimental confirmation of a theoretical prediction of a non-linear broadening of the spin packets of nitroxide free radicals due to Heisenberg spin exchange at low concentrations, C, is presented. A recent demonstration that spectra with resolved proton hyperfine structure may be analyzed efficiently and accurately was utilized to confirm the theory. As C→0, a plot of the spin-packet line width (SPW) curves downward due to the presence of proton hyperfine couplings that increase the number of distinguishable quantum spin states. At higher C, the broadening is linear with C and the results for the spin exchange rate constant determined from the slope of the broadening of the average spin-packet line width and electron spin echo measurements are in agreement. It is shown that applying modest digital smoothing does not change the values of the SPW. An example of a practical application of these methods to published work is presented, allowing an enigma to be resolved.

Analyzing the FMN-heme interdomain docking interactions in neuronal and inducible NOS isoforms by pulsed EPR experiments and conformational distribution modeling.

Nitric oxide synthases (NOSs), a family offlavo-hemoproteins with relatively rigid domains linked by flexible regions, require optimal FMN domain docking to the heme domain for efficient interdomain electron transfer (IET). To probe the FMN-heme interdomain docking, the magnetic dipole interactions between the FMN semiquinone radical (FMNH•) and the low-spin ferric heme centers in oxygenase/FMN (oxyFMN) constructs of neuronal and inducible NOS (nNOS and iNOS, respectively) were measured using the relaxation-induced dipolar modulation enhancement (RIDME) technique. The FMNH•RIDME data were analyzed using the mesoscale Monte Carlo calculations of conformational distributions of NOS, which were improved to account for the native degrees of freedom of the amino acid residues constituting the flexible interdomain tethers. This combined computational and experimentalanalysis allowed for the estimation of the stabilization energies and populations of the docking complexes of calmodulin (CaM) and the FMN domain with the heme domain. Moreover, combining the five-pulse and scaled four-pulse RIDME data into a single trace has significantly reduced the uncertainty in the estimated docking probabilities. The obtained FMN-heme domain docking energies for nNOS and iNOS were similar (-3.8kcal/mol), in agreement with the high degree of conservation of the FMN-heme domain docking interface between the NOS isoforms. In spite of the similar energetics, the FMN-heme domain docking probabilities in nNOS and iNOS oxyFMN were noticeably different (~ 0.19 and 0.23, respectively), likely due to differences in the lengths of the FMN-heme interdomain tethers and the docking interface topographies. The analysis based on the IET theory and RIDME experiments indicates that the variations in conformational dynamics may account for half of the difference in the FMN-heme IET rates between the two NOS isoforms.

High-field pulsed EPR spectroscopy under magic angle spinning.

In this work, we demonstrate the first pulsed electron paramagnetic resonance (EPR) experiments performed under magic angle spinning (MAS) at high magnetic field. Unlike nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), commonly performed at high magnetic fields and under MAS to maximize sensitivity and resolution, EPR is usually measured at low magnetic fields and, with the exception of the Spiess group work in the late 1990s, never under MAS, due to great instrumentational challenges. This hampers the investigation of DNP mechanisms, in which electron spin dynamics play a central role, because no experimental data about the latter under DNP-characteristic conditions are available. We hereby present our dedicated, homebuilt MAS-EPR probehead and show the pulsed MAS-EPR spectra of P1 center diamond defect recorded at 7 tesla. Our results reveal unique effects of MAS on EPR line shape, intensity, and signal dephasing. Time-domain simulations reproduce the observed changes in the line shapes and the trends in the signal intensity.

X-Band Single Chip Integrated Pulsed Electron Spin Resonance Microsystem.

We report on the design and characterization of a single chip integrated pulsed electron spin resonance detector operating at 9.1 GHz. The microsystem consists of an excitation microcoil, a detection microcoil, a low noise microwave preamplifier, a mixer, and an intermediate frequency (IF) amplifier. The chip area is about 0.7 mm2. To exemplify its possible applications, we report the results of single pulse, Rabi nutation, Hahn echo, two echoes, Carr-Purcell, and inversion recovery echo experiments performed on 0.02 and 0.05 nL samples of α, γ-bisdiphenylene-β-phenylallyl (BDPA) and 1% BDPA in polystyrene (BDPA:PS) at room temperature. The measured spin sensitivity is about 8 × 107 spins/Hz1/2 on a sensitive volume of about 0.1 nL. The microsystem power consumption is less than 100 mW, the radio frequency (RF) input bandwidth is 8.8 to 9.8 GHz, the IF output bandwidth is DC to 350 MHz, and the deadtime is less than 30 ns.