A great comet is a comet that becomes exceptionally bright. There is no official definition. Great comets are rare. Although comets are named after their discoverers, great comets are sometimes referred to by the year in which they appeared great, using the formulation "The Great Comet of...", followed by the year. The vast majority of comets are never bright enough to be seen by the naked eye, pass through the inner Solar System unseen by anyone except astronomers; however a comet may brighten to naked eye visibility, more it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun, a close approach to the Earth. A comet fulfilling all three of these criteria will be spectacular. Sometimes, a comet failing on one criterion will still be impressive. For example, Comet Hale–Bopp had an exceptionally large and active nucleus, but did not approach the Sun closely at all, yet it still became an famous and well observed comet.
Comet Hyakutake was a small comet, but appeared bright because it passed close to the Earth. Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across; when they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets which are making their first passage through the Solar System; the sudden brightening of comet 17P/Holmes in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet suffered a sudden outburst which caused it to brighten by factor of about half a million, it unexpectedly brightened from an apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye. All these temporarily made comet 17P the largest object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter.
The brightness of a simple reflective body varies with the inverse square of its distance from the Sun. That is, if an object's distance from the Sun is halved, its brightness is quadrupled. However, comets behave differently, due to their ejection of large amounts of volatile gas which also reflect sunlight and may fluoresce, their brightness varies as the inverse cube of their distance from the Sun, meaning that if a comet's distance from the Sun is halved, it will become eight times as bright. This means that the peak brightness of a comet depends on its distance from the Sun. For most comets, the perihelion of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within 0.5 AU or less may have a chance of becoming a great comet. For a comet to become spectacular, it needs to pass close to the Earth if it is to be seen. Halley's Comet, for example, is very bright when it passes through the inner Solar System every seventy-six years, but during its 1986 apparition, its closest approach to Earth was the most distant possible.
The comet was unspectacular. On the other hand, the intrinsically small and faint Comet Hyakutake appeared bright and spectacular due to its close approach to Earth at its nearest during March 1996, its passage near the Earth was one of the closest cometary approaches on record. Great comets of the past two millennia include the following: The bright-comet chronicles. John E. Bortle Memorable Comets of the Past Gary W. Kronk. Brightest comets seen since 1935
The Buchwald–Hartwig amination is a chemical reaction used in organic chemistry for the synthesis of carbon–nitrogen bonds via the Palladium-catalyzed coupling reactions of amines with aryl halides. Although Pd-catalyzed C-N couplings were reported as early as 1983, Stephen L. Buchwald and John F. Hartwig have been credited, whose publications between 1994 and the late 2000s established the scope of the transformation; the reaction's synthetic utility stems from the shortcomings of typical methods for the synthesis of aromatic C–N bonds, with most methods suffering from limited substrate scope and functional group tolerance. The development of the Buchwald–Hartwig reaction allowed for the facile synthesis of aryl amines, replacing to an extent harsher methods while expanding the repertoire of possible C–N bond formation. Over the course of its development, several'generations' of catalyst systems have been developed, with each system allowing greater scope in terms of coupling partners and milder conditions, allowing any amine to be coupled with a wide variety of aryl coupling partners.
Because of the ubiquity of aryl C-N bonds in pharmaceuticals and natural products, the reaction has gained wide use in synthetic organic chemistry, with application in many total syntheses and the industrial preparation of numerous pharmaceuticals. The first example of a palladium catalyzed C–N cross-coupling reaction was published in 1983 by Migita and coworkers and described a reaction between several aryl bromides and N,N-diethylamino-tributyltin using 1 mol% PdCl22. Though several aryl bromides were tested, only electronically neutral, sterically unencumbered substrates gave good to excellent yields. In 1984, Dale L. Boger and James S. Panek reported an example of Pd-mediated C–N bond formation in the context of their work on the synthesis of lavendamycin which utilized stoichiometric Pd4. Attempts to render the reaction catalytic were unsuccessful; these reports were uncited for a decade. In February 1994, Hartwig reported a systematic study of the palladium compounds involved in the original Migita paper, concluding that the d10 complex Pd2 was the active catalyst.
Proposed was a catalytic cycle involving oxidative addition of the aryl bromide. In May 1994, Buchwald published an extension of the Migita paper offering two major improvements over the original paper. First, transamination of Bu3SnNEt2 followed by argon purge to remove the volatile diethylamine allowed extension of the methodology to a variety of secondary amines and primary anilines. Secondly, the yield for electron rich and electron poor arenes was improved via minor modifications to the reaction procedure, although no ortho-substituted aryl groups were included in this publication. In 1995, back to back studies from each lab showed that the couplings could be conducted with free amines in the presence of a bulky base, allowing for organotin-free coupling. Though these improved conditions proceeded at a faster rate, the substrate scope was limited entirely to secondary amines due to competitive hydrodehalogenation of the bromoarenes; these results established the so-called "first generation" of Buchwald–Hartwig catalyst systems.
The following years saw development of more sophisticated phosphine ligands that allowed extension to a larger variety of amines and aryl groups. Aryl iodides and triflates became suitable substrates, reactions run with weaker bases at room temperature were developed; these advances are detailed in the Scope section below, the extension to more complex systems remains an active area of research. The reaction mechanism for this reaction has been demonstrated to proceed through steps similar to those known for palladium catalyzed C-C coupling reactions. Steps include oxidative addition of the aryl halide to a Pd species, addition of the amine to the oxidative addition complex, deprotonation followed by reductive elimination. An unproductive side reaction can compete with reductive elimination wherein the amide undergoes beta hydride elimination to yield the hydrodehalogenated arene and an imine product. Throughout the development of the reaction the group sought to identify reaction intermediates through fundamental mechanistic studies.
These studies have revealed a divergent reaction pathways depending on whether monodentate or chelating phosphine ligands are employed in the reaction, a number of nuanced influences have been revealed. The catalytic cycle proceeds as follows: For monodentate ligand systems the monophosphine palladium species is believed to form the palladium species, in equilibrium with the μ-halogen dimer; the stability of this dimer decreases in the order of X = I > Br > Cl, is thought to be responsible for the slow reaction of aryl iodides with the first-generation catalyst system. Amine ligation followed by deprotonation by base produces the palladium amide; this key intermediate reductively eliminates to regenerate the catalyst. However, a side reaction can occur wherein β-hydride elimination followed by reductive elimination produces the hydrodehalogenated arene and the corresponding imine. Not shown are additional equilibria wherein various intermediates coordinate to additional phosphine ligands at vario
Sailing Soul is the debut mixtape by American singer-songwriter Jhené Aiko. Aiko began working on the mixtape after she gave birth to her daughter, Aiko wrote all the mixtape's songs except "July,", written by Micah Powell. Most songs on the mixtape were produced by Fisticuffs, except “July” and “You vs Them” which were produced by Bei Maejor, “Real Now”, produced by Roosevelt, “Do Better”, produced by J. Lbs and “Growing Apart”, produced by Tae Beast; the mixtape featured several guest vocalists, including Miguel, Gucci Mane, Kanye West, Kendrick Lamar as well as others. Critically the mixtape received positive reviews, with reviewers saying that the mixtape was one of the "freshest", another reviewer said that the mixtape is making "Noise in the R&B spectrum"; the concept follows the story of Aiko having a meeting with a label and she was told by people at the label "I love everything but when you come into these meetings you have to sell yourself." After the meeting Jhene came up with the concept "I’m not a slave to anyone.
The free tickets were given away at the Key Club box office on July 6 from 11AM-5PM on a first-come, first-served basis while. The concert finished as 9:30 pm, lasting a duration of two and a half hours. On October 21, 2012 Aiko released a music video for the song "My Mine". Jhene released a video for the mixtapes second track "Stranger". Datpiff certified the mixtape Platinum with downloads of 300,000 and an additional 31,000 downloads from MixtapeMonkey.com. In 2002, Jhené made her musical beginning when she featured on several B2K, she featured on tracks 5 through 8 on B2K's B2K: The Remixes - Volume 1, "My Name Is Jhene", "He Couldn't Kiss", "Gonna Love You Anyway", "Stuck Like This". "Santa Baby", a cover she recorded, was featured on B2K's Santa Hooked Me Up. are sung by her. On Pandemonium!, she was featured as a guest vocalist on the song "Tease", her song "Dog" appeared as a bonus track on B2K's "Pandemonium". The full version of "Stuck Like This" can be heard as the final track on The Remixes - Volume 2.
Jhené has appeared in numerous music videos including O'Ryan's debut video, "Take It Slow", B2K's debut video Uh Huh and "Why I Love You", P. Y. T.'s Same Ol' Same Ol' featuring Sarai, Play's "M. A. S. T. E. R." Featuring Lil' Fizz video and Morgan Smith's 2004 video Blow Ya Whistle. Jhené has songs featured on the soundtracks of "Barbershop", "The Master of Disguise", "You Got Served", "The Proud Family" and "Byou", she released a video for her single "NO L. O. V. E" which debuted on BET Park when she was 15 years old. Jhene was set to release her self-titled debut album but the album was never released due to tension at the record label Epic, in which Aiko asked to be released from the label. Aiko began to focus on school. At the age of 20, Aiko became pregnant by American R&B singer O'Ryan and gave birth to a baby girl named Namiko Love Browner on 19 November 2008. After Aiko gave birth to her daughter, six months she began to work and develop on the mixtape. Aiko stated that she was going through trouble at this period in her life saying "At the time I was going through so much in my life that it was so easy to write the songs.
I had just had a baby and I was dealing with baby father issues and I had a boyfriend. The songs were just coming. I just had so much to say, when I was signed, I never got a chance to write my own music. I always wanted to do a mixtape and before I got pregnant I was about to get back into it ‘cause I was working with different producers and things like that. Fisticuffs, who produced the majority of sailing soul, I’ve known them for a while and worked with them on different projects. So yeah, the concepts were just coming and it only took about nine months to complete". All the songs on the mixtape were written by Aiko, except “July”, written by Micah Powell and most of the songs were produced by Fisticuffs, except “July” and “You vs Them” which were produced by Bei Maejor, “Real Now”, produced by Roosevelt, “Do Better”, produced by J. Lbs and “Growing Apart”, produced by Tae Beast; the album's artwork was shot by photographer Jay 3. The mixtape features collaborations from Miguel and Kanye West as well as others.
In 2011, she began working with artists from Carson-based independent record label Top Dawg Entertainment. Aiko described the concept of this mixtape during an interview with Vibe Magazine, in which she said "The concept came from the meeting right before I found out I was pregnant. I had a meeting with this label head and at this point I had done so many meetings. I had been doing it for like five years, I was excited to have the meeting, but it wasn’t hype. I was going to go in there and be myself, I’m not going to care. I wore a little bit of makeup and I dressed how I would dress. Went in there, sang for them, did the whole meeting thing and he was like, ‘I love your voice, I love everything but when you come into these meetings you have to sell yourself.’ I was just speechless when he said that, I have no filter when it comes to speaking my mind so I bit my tongue so hard. I wanted to say, ‘No! I don’t have to sell myself. I am me.’ You like me or you don’t. After that, I found out I was pregnant and I was on Twitter one day and said something about selling your soul, but I spelled it wrong and Chase N. Cashe corrected me.
I put “sailing souls” and he said, ‘Oh, that would be a nice name for an album.” And everything started