A single-level random-effects cross-lagged panel model for longitudinal mediation analysis

Cross-lagged panel models (CLPMs) are widely used to test mediation with longitudinal panel data. One major limitation of the CLPMs is that the model effects are assumed to be fixed across individuals. This assumption is likely to be violated (i.e., the model effects are random across individuals) in practice. When this happens, the CLPMs can potentially yield biased parameter estimates and misleading statistical inferences. This article proposes a model named a random-effects cross-lagged panel model (RE-CLPM) to account for random effects in CLPMs. Simulation studies show that the RE-CLPM outperforms the CLPM in recovering the mean indirect and direct effects in a longitudinal mediation analysis when random effects exist in the population. The performance of the RE-CLPM is robust to a certain degree, even when the random effects are not normally distributed. In addition, the RE-CLPM does not produce harmful results when the model effects are in fact fixed in the population. Implications of the simulation studies and potential directions for future research are discussed

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH3_3CHCH2_2O)

Life on Earth relies on chiral molecules, that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH$_3$CHCH$_2$O), in absorption toward the Galactic Center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found

Hydrogen bonding in the ethanol–water dimer

We report the first rotational spectrum of the ground state of the isolated ethanol–water dimer using chirped-pulse Fourier transform microwave spectroscopy between 8–18 GHz. With the aid of isotopic substitutions, and ab initio calculations, we identify the measured conformer as a water-donor/ethanol-acceptor structure. Ethanol is found to be in the gauche conformation, while the monomer distances and orientations likely reflect a cooperation between the strong (O–H⋯O) and weak (C–H⋯O) hydrogen bonds that stabilizes the measured conformer. No other conformers were assigned in an argon expansion, confirming that this is the ground-state structure. This result is consistent with previous vibrationally-resolved Raman and infrared work, but sheds additional light on the structure, due to the specificity of rotational spectroscopy

Hydrogen bond competition in the ethanol–methanol dimer

Previous theoretical work on the ethanol–methanol dimer has been inconclusive in predicting the preferred hydrogen bond donor/acceptor configuration. Here, we report the microwave spectrum of the dimer using a chirped pulse Fourier transform microwave spectrometer from 8–18 GHz. In an argon-backed expansion, 50 transitions have been assigned to a trans-ethanol-acceptor/methanol-donor structure that is likely stabilized by a secondary weak C–H⋯O hydrogen bond. A higher energy conformer was observed in a helium-backed expansion and tentatively assigned to a gauche-ethanol-acceptor/methanol-donor structure. No ethanol-donor/methanol-acceptor dimers have been found, suggesting such interactions are energetically disfavored. A preliminary analysis of the A–E splitting due to the internal rotation of the methanol methyl group in the ground state species is also presented. We find evidence of the Ubbelohde effect in the measured A–E splittings of three deuterated isotopologues and the normal species of this conformer

Geochemical impact of urban development on fragile freshwater wetlands

Urbanization is associated with increased cover of impervious surfaces, which poses significant challenges to freshwater ecosystems globally. Implications of catchment urbanization include altered natural hydrology, erosion, weed invasions and modified water chemistry. Blue Mountains Upland Swamps are sensitive freshwater ecosystems located in the Blue Mountains region in south-eastern Australia. They have high conservation value as they are located within a World Heritage Area, are a listed 'endangered ecological community' in Australia and contain endemic and endangered flora and fauna. Water chemistry was assessed in four naturally vegetated and four urban swamps. Urban swamps had higher impervious cover and modified water chemistry, with elevated pH, electrical conductivity and major ions compared to non-urban swamps. Water in urban swamps had elevated calcium, potassium and bicarbonate compared to non-urban catchments, by 19.8, 5.2 and 10.3 times respectively. Although further research is needed, we hypothesize that common concrete materials, particularly drainage infrastructure, strongly influenced differences between urban and nonurban catchments. This adds to growing international research highlighting the potential role of concrete in modifying urban water chemistry due to gradual dissolution and mobilization of ions. In an increasingly urban world, consideration of the ecological consequences of urbanization is required to guide future management approaches

Signatures of urbanization in Temperate Highland Peat Swamps on Sandstone (THPSS) of the Blue Mountains World Heritage Area

Urban freshwater ecosystems exhibit distinct patterns of elevated major ions (calcium, potassium and bicarbonate) and metals, referred to as the ‘urban geochemical signature’. However, the implications of this urban fingerprint at the water-sediment interface within sensitive freshwater systems are not well-known. Temperate Highland Peat Swamps on Sandstone are unique freshwater wetlands found within and surrounding the high-conservation value Greater Blue Mountains World Heritage Area and are a listed ‘endangered ecological community’ in Australia. Water and sediment chemistry were assessed within four urban and four naturally vegetated swamp catchments, through field monitoring and novel laboratory techniques (including X-ray diffraction). Urban swamps had distinct elemental signatures compared to naturally vegetated swamps. Urban swamp water displayed increased pH, elevated ionic strength, major ions (calcium and bicarbonate) and metals (strontium, barium, manganese and iron). Urban swamp sediment had higher calcium, with calcium hydroxide detected at two urban sites. Urban development and concrete drainage infrastructure in swamp catchments modify natural hydrology and water chemistry. Findings suggest swamp sediments may act as sinks of metals and alkalinity, with urbanization remaining a potential source. However, the consequences for high-conservation value systems are not well understood. As urbanization continues to expand, this has implications for fragile freshwater environments worldwide

Identification of two conformationally trapped n-propanol-water dimers in a supersonic expansion

Two conformers of the n-propanol-water dimer have been observed in a supersonic expansion using chirped-pulse Fourier-transform microwave (CPFTMW) spectroscopy. Structural assignments reveal the n-propanol sub-unit is conformationally trapped, with its methyl group in both Gauche and Trans orientations. Despite different carbon backbone conformations, both dimers display the same water-donor/alcohol-acceptor hydrogen bonding motif. This work builds upon other reported alcohol-water dimers and upon previous work detailing the trapping of small molecules into multiple structural minima in rare gas supersonic expansions

Decade-Spanning High-Precision Terahertz Frequency Comb

The generation and detection of a decade-spanning terahertz (THz) frequency comb is reported using two Ti:sapphire femtosecond laser oscillators and asynchronous optical sampling THz time-domain spectroscopy. The comb extends from 0.15 to 2.4 THz, with a tooth spacing of 80 MHz, a linewidth of 3.7 kHz, and a fractional precision of 1.8×10^(−9). With time-domain detection of the comb, we measure three transitions of water vapor at 10 mTorr between 1–2 THz with an average Doppler-limited fractional accuracy of 6.1×10^(−8). Significant improvements in bandwidth, resolution, and sensitivity are possible with existing technologies

A decade-spanning high-resolution asynchronous optical sampling terahertz time-domain and frequency comb spectrometer

We present the design and capabilities of a high-resolution, decade-spanning ASynchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ∼80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region

Experimental evidence that evolutionary relatedness does not affect the ecological mechanisms of coexistence in freshwater green algae

The coexistence of competing species depends on the balance between their fitness differences, which determine their competitive inequalities, and their niche differences, which stabilise their competitive interactions. Darwin proposed that evolution causes species' niches to diverge, but the influence of evolution on relative fitness differences, and the importance of both niche and fitness differences in determining coexistence have not yet been studied together. We tested whether the phylogenetic distances between species of green freshwater algae determined their abilities to coexist in a microcosm experiment. We found that niche differences were more important in explaining coexistence than relative fitness differences, and that phylogenetic distance had no effect on either coexistence or on the sizes of niche and fitness differences. These results were corroborated by an analysis of the frequency of the co‐occurrence of 325 pairwise combinations of algal taxa in > 1100 lakes across North America. Phylogenetic distance may not explain the coexistence of freshwater green algae.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/100307/1/ele12182.pd